What was the reference in the post credit scene in Guardians of the Galaxy?

There is a little bit of further history to add to this, as for Marvel Fans the inclusion of Howard the Duck is partially contentious...

Up until very recently, Howard the Duck has been a largely off-topic point of discussion for Marvel (where the polite etiquette is to keep your mouth shut about it). To discuss Howard would be akin to congratulating Pixar for John Carter of Mars; it caused a lot of trouble/stress, and everyone would much rather not talk about it.

Howard the Duck was a character created by Steve Gerber, a highly respected Marvel writer who passed away from a long-running illness. Steve Gerber famously struggled with Marvel for creative control of the character, as he (and Howard the Duck) are at the centre of a controversy that fundamentally affected the concept of 'Creator-Owned' characters in comics.

Despite being the sole creator of Howard the Duck, Gerber was denied scalable pay for the amount of income Marvel was receiving from the line. When Gerber tried to take the character to another, independent publishing company; he discovered that he had waived his legal right to any claim over the character by the terms of his contract with Marvel.

Gerber left Marvel embittered, and created a deliberate (and, as Marvel lawyers argued, copyright infringing) duplicate of Howard the Duck: Destroyer Duck. Howard the Duck has been a bitter affair for Marvel ever since, and their attempts (by George Lucas) to turn the property into a movie fell flat: although Gerber was happy to see his creation realized cinematically, and even attending the set of the Movie on numerous occasions.

The way Gerber was treated was a real wake-up call for Comic Book artists across the industry, and became the rallying cry (along with examples from the likes of Steve Ditko and Jack Kirby) for a mass-exodus of talent from Marvel...

Spawn comics are a result of this, as Todd Macfarlane made a speech at Mark Gruenwald funeral directly criticizing Marvel for its treatment of the Bullpen, and citing Gerber/Howard the Duck as soapbox example of everything that was wrong with Marvel.

A few months after Gruenwald died, Steve Gerber took his revenge against Marvel for their treatment of what he saw as his baby Duck.

In a special crossover series of Savage Dragon/Destroyer Duck written by Gruenwald, we see Destroyer Duck 'Save' what is unmistakably (but not overtly confirmed for obvious litigious reasons) Howard the Duck from his environment, claiming he has swapped him for a inferior clone/robot version of him, of a lesser quality.

For the comic community, it was a daring Coup-de-état against Marvel: Gerber was given the 'privilege' of writing a cameo part of the Marvel Universe, and used the opportunity to alter the lore of the MU by reclaiming his personal creation, and casting aspersions over the validity of any future use of the character by Marvel... and because the series was produced in issues alternating in publication between Marvel and Image (with this issue being an Image one), Marvel were unable to do anything (editorially, at least) about it.

In short, Gerber screwed Marvel over, and Howard the Duck is a by-word for the era of a non-unified, at odds with itself Marvel of the past.

The fact is, since the Toybiz merger of the mid-90's conditions improved dramatically for Marvel, and by the time Disney had bought them out as an entertainment company they had restored to a (largely) cohesive and co-operative organisation.

However: even when Marvel merged with Disney, Howard the Duck had one last spanner to throw into the works:

Due to Howard's resemblance to Donald, Disney had almost sued Marvel for copyright infringement until Howard was given a redesign to ease the dispute (hence his wearing pants).

Howard has since appeared as a character in the Disney/Marvel Animation properties; Ultimate Spider-Man, Phineas and Ferb: Mission Marvel and Hulk and the Agents of S.M.A.S.H.

His cameo in Guardians of the Galaxy is part of an ongoing 'celebration' by Marvel/Disney that recognizes that their past differences are behind them. Howard is even featured/parodied in Pixars' Planes 2: Fire and Rescue as 'Howard the Truck'.

In summary: to anyone familiar with the largely famous ongoing disputes surrounding Howard the Duck, his inclusion in a MCU film is not only a gesture of respect to Gerber's legacy, but an indication that they have moved on from their largely slef-destructive treatment of their fans.

This is from Geek Tyrant, who lifted an Empire interview with the film's director, James Gunn:

"I think it was some combination of me and the editor Fred Raskin who said, ‘Let’s put Howard The Duck in there. What if The Collector looks over and sees Howard The Duck sitting there?’ And I wrote down the line, ‘Whaddya let it lick you like that for? Gross.’ Fred and I thought it was hilarious, but we weren’t sure that (Head of Marvel Studios) Kevin Feige would go for it. But we told Kevin and Kevin couldn’t stop laughing, so that’s how it came about."

When asked about Howard the Duck possibly getting his own movie, Gunn replied:

"I’ll be honest with you, I was just talking about it with my assistant right now (Laughs). It’s possible Howard could reappear as more of a character in the Marvel Universe. But if people think that’s going to lead to a Howard The Duck movie, that’s probably not going to happen in the next four years. Who knows after that?"

So, the duck is called Howard.

To give a little more context, Howard the Duck was a 1986 sci fi movie which was, according to its wiki, the first attempt at a theatrical release [of a film based on a Marvel character] since the Captain America serial of 1944.

It was included in the post-credits of Guardians of the Galaxy as a homage to that first Marvel outing.