We know that Sirius Black, James Potter, McGonagall, and Pettigrew are animagi. Is Dumbledore an animagus or are there any other animagus characters mentioned in the Harry Potter series?
Dumbledore actually helped McGonagall becoming an animagus when she was his student, according to JKR on Pottermore. This might suggest that he himself was an animagus.
However, there is no canon evidence to support this theory. Additionally Dumbledore would have no reason (in my opinion) to keep this a secret. So if he was one, it would most likely be known to the ministry (and thus to us).
Not very likely.
All animagi in canon1 are clearly identified2. Of course, anyone could be or have been an animagus and just not have used the ability, but there is no reason to believe there are any unidentified animagi3.
While Dumbledore was a transfiguration teacher prior to becoming headmaster and tutored McGonagall, there is no evidence of him having become one.
1: Of course, in fan fiction, anyone can turn out to be an animagus indeed, usually to compy with rule 34.
2: Minerva McGonagall: cat, registered; James Potter (deceased): stag, unregistered; Sirius Black (deceased): dog, unregistered; Peter Pettigrew (deceased): rat, unregistered; Rita Skeeter: bug, unregistered.
3: Hermione once grew some whiskers, but that was because of a polyjuice mishap. And Draco turned into a ferret once, but not through his own volition.