What exactly is Billy Cole?

According to the Fright Night wiki, he was a vampire servant:

A vampire servant is someone who has been bitten by vampire and made into a loyal protector. They are not made into full vampires, but they have great strength and seem to survive nearly everything a vampire can. They can survive in the sunlight like an ordinary person.

While there is a clear distinction between vampires and the more-than-human ghouls that serve them it could be said that the vampires that are recently turned by older, more powerful ones like Jerry or Regine Dandridge are beholden to them. This master/ disciple relationship makes the younger vampires the thralls of their master's will much like the mortal ghouls Billy Cole and Bozworth.

Given the paragraph above, if given the choice I think I'd rather be the servant!

It is worth noting however that the above paragraph doesn't reference anything. The question was also asked over at the IMDB in the FAQ for the film, with the answer stating:

At one point, Peter Vincent notes that Billy can walk about in the daylight and proclaims that he is human. Near the end of the movie, however, Billy disintegrates into sand and green goo, definitely not human. Most viewers conclude that Billy was some kind of ghoul or possibly a 'half-vampire' who hasn't yet been fully turned. Another possibility is that he was a golem, made by Dandrige from sand and other matter in order to serve as his protector.

This seems to suggest that there is no definitive answer.

I like the idea of the vampire servant and how he has only been partly turned as it appears to draw similarities with characters like Renfield and Jonathan Harker in Dracula - being subservient and yet not fully turned.

However - it is never explicitly stated in the film and hasn't been explicitly addressed since. The most common answer seems to be a ghoul/familiar (i.e. half turned servant), with the only certainty about him being that he isn't a vampire as he can withstand daylight.

If have to say a golem, a ghoul is a fiend of type who participates with the dead, eating corpses, grave robbing and necrophelia. Where as being a Golem, (obviously not traditional, Jewish lore) he's to serve and protect his master. Also, a half Vampire wouldn't have that effect in death, rather that of a typical human or vampire depending how far in transformation he or she is.

Billy was a human servant of a vampire. When bitten he is not fully drained of blood but is given vampires blood as a result he has enhanced strengh, can walkabout in the daytime and can as long as his master unless killed if killed he can return as a full vampire due to the vamp's blood in his system but his primary function is to protect his master during the day or take care of his daytime affairs if ordered to he will bring unwilling victims to his masters lair.