Who fired the shot and what is the symbolism in firing shot right next to Lucas at the end of the movie?

There are no right answers to this question. The director has refused to comment on the issue, simply saying it is open to interpretation. Therefore, in this very long answer I've tried to address a whole range of topics to hopefully give some indication as to my subjective interpretation of events.

If you want a summary: We have no way of knowing who the shooter is, but I believe it to be irrelevant - it is simply someone showing Lucas that he and his family will never truly be accepted in the community.

To begin the long answer to this question, I believe you need to understand how important the court of public opinion is. People will often make up their own beliefs on a trial, regardless of what facts or knowledge have been presented to them. This is evident regarding any type of trial. Look at how vociferously people will defend or attack the likes of Kobe Bryant, Michael Jackson, the parents of Madeleine McCann, often with little knowledge of the actual events that have transpired. - the list just goes on.

In The Hunt, Lucas is wrongfully accused of child molestation. Despite little evidence, he is shunned by his entire community. He loses his friendship with his best friend, his son becomes an outcast at school and he breaks up with his girlfriend. His own ostracism in the community becomes violent as the movie tragically shows. When he is found to be innocent, we skip forward a year to the ending of the film and learn that the community has accepted him and his family again. The scene with his son being inducted into the local hunting society is supposed to symbolise their reintegration into society.

But - None of this changes the fact he was accused of a heinous crime. For many people, the mere implication he was involved will be enough to tarnish his reputation in their eyes and forever label him a paedophile. The scene where the son is inducted shows this. Mads Mikkelson, the actor who played Lucas, discussed this at length in an interview with Collider:

The fact that this man didn’t do what he’s being accused of is pretty clear, but this film doesn’t really give a definitive closure to the story. Were you okay with that? Did you come to your own conclusions about the ending?

MIKKELSEN: For me, it was just important to know whether he did it or not. As you say, you can unquestionably see that he hasn’t done it. You see why the little girl is doing what she’s doing. You understand everything. And that was very important for me to know. If it was the other case and he had done it, I would have still done the film. I just would have made sure that we knew he had done it, and show it from the other angle. The finish of the film is obviously a metaphor, or it can be real. But, in the scene just prior to that one, where his son is getting handed this rifle, he’s looking over at all of his friends. It’s the only stylistic scene in the film. They’re all looking at him in strange ways that are unreadable. You can’t read what they are thinking. And then, it snaps back to the party and his son gets his rifle and it’s all good. But, that is the moment where he realizes that this is not going to happen. He can’t stay there. It doesn’t matter how much they try to accept him. They will never be able to accept him, fully. That is obviously the shot at the end. It’s the icing on the cake, in the sense that he can’t stay there. He’s gotta move on.

He clearly believes the son (and family) will never be accepted in the community.

To look at why they may not be accepted, consider looking at the situation from a different angle. Here is some advice from the "Child Safety For Parents" website on the "profile of a paedophile":

Tip #1: Watch His Behavior
Is he interested more in kids than adults?
Does he offer to baby-sit or take the kids to give you a break?
Does he work closely with kids?
Is he especially affectionate toward kids?
Does he have activities with kids when parents are not invited or involved?

This is just one example from one website, but as you can see - many of their suggestions are applicable to almost anyone in the primary education sector (especially someone like a tutor who would regularly have activities with students without parental supervision). This doesn't mean the advice is wrong, but rather it's so broad it can apply to any number of people and thus can be very dangerous advice if not applied appropriately. In a small town like this film is set in, when one person gets accused of something as heinous as this, suddenly all their actions are analysed and judged. Everything they've done. And there is always something that can be used to suggest wrongdoing. Every small act of kindness and support Lucas ever offered to any child could be used as evidence against him.

Additionally, it must be remembered that the mere mention of paedophilia is enough to blind some people into acting without logic. A phenomenal (and scary) example of this was a famous attack in 2000 in the UK (discussed here) where a paediatrician (i.e. doctor) had her house covered in paint and graffiti due to her acts of child molestation - the culprits hadn't realised there was a difference between paediatrician and paedophile.

Going back to the movie, I think another very important issue to comprehend is why Klara made the false claim?

The Quietus discuss the idea of the sexualisation of society being the culprit in their review of the film:

The phrase 'a random lie' seems to be the accidental key to the film. These words, which fade onto screen in the trailer, are set against the image of the young protagonist Klara (played by Annika Wedderkopp) - the 'innocent' source of the random lie – a lonely girl, lacking in family intimacy and attention, who accuses Lucas (Mads Mikkelsen) – her father's best friend and also her kindergarten teacher – of sexual abuse. Her accusation is ambiguous and confused, and Vinterberg's skill as a director of group drama comes into play so clearly in his orchestration of the suspicion and its poisonous and often violent reactions in the people that surround her. Lucas becomes, almost immediately, a pariah in his own community. The people around Klara - the police, child psychologists, teachers, friends, sibling - are frantic to convince her of the veracity of her accusation, and in doing so 'fill in' her imagination until she is unsure of her own mind; unable to distinguish memory from fiction.

Yet nothing about her lie is random, and so in one way the trailer advertises a narrative fallacy. For what The Hunt is really about is the absolute sexual confusion that exists within the community and, by extension, in modern European society. How can Klara's lie be 'random', when just before she makes her accusation, her older brother and his friend run through the house and show her a picture of a woman engaged in a sex act on their mobile phones? It doesn't take much to assume a wider context around Klara, for it surrounds all of us. Young girls are being coaxed into early sexualisation through advertising, movies, dolls, magazines, television and pop music. Vinterberg's film adeptly makes the point that Klara's lie is a result of confusion – a leakage of a kind of tectonic ambiguity which exists beneath contemporary society, muddying the relations between adults and children.

The Australian also discuss this at length in their review of the film:

Klara is not a vindictive child; she has a generous nature. But she lives in a community where boisterous male bonding rituals are not uncommon, where the men (Lucas included) like to strip naked and take icy dips in winter waters, where male erections may be glimpsed at odd moments, and where children such as Klara can easily be exposed to explicit sexual images on an iPad.

One day, after some romping and horseplay with a group of children, Klara plants a kiss on Lucas's lips. We take it to be a loving gesture. Then, out of peevishness, she takes offence when Lucas turns down an offered gift. But she doesn't rush off to the kindergarten principal (Susse Wold) to accuse Lucas of sexual abuse or indecent exposure.

Grethe, the principal, a kindly and sensible woman, nevertheless suspects that something is troubling Klara. Under questioning, the child will say nothing against Lucas, but Grethe isn't satisfied. A counsellor is brought in and the girl is questioned relentlessly. Are you sure Lucas didn't do this? Are you sure he didn't show you his willie? Eventually, to please her inquisitors, Klara gives a little nod of the head. It's enough; the police are called, Lucas is arrested and questioned. But even when Klara confides later (to her mother, to Grethe, to Lucas himself) that "I just said something foolish", no one wants to believe her. Soon stories are coming in from other parents reporting "symptoms of abuse" in their children - though what these symptoms are is never clear.

So it can easily be seen from both film reviews that the film plays on the idea of a community being so scared of something that they end up seeing it everywhere. They fear Lucas is guilty and need to know the truth - and thus end up generating the opposite of that from the children with their leading questions. The community effectively comes together (wrongly) to protect their children and themselves. Whilst Lucas' eventual innocence heals some wounds, this protective layer that has been raised against him and his family can never be truly gone.

Now, after all that, it's time to specifically address your actual questions (I hadn't forgotten!). It isn't revealed who the shooter is. It could be anyone (or in fact no one at all).

It's certainly plausible that the scene is imagined. After all, why does the man not simply reload and kill him? This would fit in with the idea that for the rest of his life he'll be paralysed by fear (in fact, this is very similar to the ending of Misery, where James Caan realises he'll be haunted for life by his experiences).

Alternatively, it could be the scene is real and perhaps the shooter either realises how close he came to murder and is scared by his own actions. Or, and more likely, he (or she) really just wants to send a warning message, that Lucas will never be safe. Perhaps the person feels they are above that of murderer and are simply trying to scare this evil presence out of their community.

Given that there is no credible resource confirming the ending one way or another (as it would take away from the power of its open ended nature), it's up to us as viewers to draw our own conclusions. It could be argued it is Lucas' son, angry at all the suffering and abuse he had to put up with, who fires the shot. But that seems highly improbably given the fact all the abuse and suffering is starting to end and he is finally part of the society. Why now? Possibly he sensed he was still an outcast at the end of the film and chose to blame his father, but to me it wouldn't make sense. It certainly is plausible that it's Klara's brother. We see him crying at one point in the film in anger and sadness over his sister's supposed attack. Perhaps guilt drives him (realising he showed her sexual images too at her young age), or perhaps more likely it was simply anger blind to the truth that led him to his actions.

Ultimately, we have no way of knowing. With that in mind, this is my conclusion of events.

The scene is real. Someone from the court of public opinion (whether Klara's brother or a random towns-person is irrelevant) has refused to believe Lucas is innocent and has effectively threatened him, warning him he and his family will never be "part" of the community. His experience made him an outcast and there's truly no way back from that.

This is what Thomas Vinterberg had to say about the issue in an interview with Slant when discussing the nature of the townspeople.

People in The Hunt aren't monsters, yet the things they do are evil.

I consider them all good. I find that they're all innocent, sweet, and pure people who have this splinter in the eye that takes away their innocence. What I consider really sad about this film, and what touches me about it, is that somehow it became a reflection on loss of innocence in the world. I grew up in the '70s, and back then, in my childhood imagination, people were naked and good. It was possible to be physically naked around each other without being put to prison. Everything was orange, and with time, things have become more blue. For good reasons obviously: We now know that children are being abused, so there's a good reason for all this. But we have lost something along the way, something that I find very dear.

Effectively, the townspeople panic. Cineoutsider discussed this as well, although with much less sympathy for the townspeople:

There's only one villain here, and it isn't Lucas, but it isn't Klara either. It's the townspeople. The Hunt is Hawthorne's The Scarlet Letter and Miller's The Crucible for the modern age, with both works serving as an inspiration and subtle presence, just as they were for von Trier in Dogville. Panic and hysteria spreads through the town like a virus. Their desperation is palpable, and the speed at which Lucas' life is destroyed is terrifying. The shock value – and perhaps the sensationalism – of The Hunt comes in witnessing the behaviour of the townspeople, and their vigilante, lynch mob mentality. Under the guise of protecting their town and their children, their vitriolic behaviour is accepted as normal, and Lucas continues to be ostracised. There are times, particularly towards the end of the film, where things veer toward the melodramatic, when all that Lucas suffers just seems that bit too much, and it's difficult to imagine when and where it might stop. Injustice seems too small a word to describe all that befalls him. Thankfully, the film remains grounded, stopping short of turning him into a martyr, but only just. There are moments when it feels like Lucas could meet a similar end to The Wicker Man's Sergeant Howie and no one would so much as blink.

The last line, describing how Lucas could end up similar to the iconic Wicker Man ending is particularly horrifying given how plausible it is.

On a final note - think of the title of the movie. The Hunt. For Lucas, the Hunt will never truly end. He will always be targeted for his supposed sins and the court of public opinion will never leave him be.

It was Marcus. Look at the facts: Marcus was in the middle of a horrible divorce with a castrating mother yet still stood up for his father. He went to his father to show his support. Lucas brushed him off when he tried to tell him that he was #1 gay, #2 not interested in the hunt. Marcus didn't fit in and related to the ignorance and hate his father was experiencing. Then, "THEY" killed his beloved dog. Would you forgive a group of people who killed your dog? Seriously? Then, fa la la, all is fine when "Marcus" becomes a real man. Marcus promised to kill every single one of them, every one. Now, he's furious at his dad for forgiving those people who ruined both of their lives. Lucas still doesnt see the rage that is inside his son and, instead, subjects him to a party with those same people that Marcus hates. Now Marcus gets his revenge and shoots a warning shot, a "remember my promise " shot at his dad, which is why he doesn't kill him and walks away. He is letting his weak dad know that HE has not forgiven this town of idiots nor the child that made up the story. Lucas is gentle and forgiving; Marcus is not and feels betrayed. The next movie will be "the Marcus Massacre".

To me it symbolises that once you've been falsely accused, you can never completely wash it away and some of the damage is permanent.

The penultimate scene, where they celebrate Marcus, is upbeat and it seems that all wrongs have been righted, but the ending suggests that this is not completely true and some of the damage cannot be undone.

Whoever fires the shot still harbours ill will towards Lucas, which suggests there are people out there who are not completely convinced of his innocence.