that, but it does seem that pretty much anyone
can be an animagus if it suits the story
I believe the only registered animagus we've come across in the Harry Potter books is Professor McGonagall. However, Sirius Black, Harry's father James Potter, Peter Pettigrew (Wormtail), and the tabloid journalist Rita Skeeter are animagi as well. The impression is given in the first book or two that being an animagus is a gift, you either are, or you aren't. However, James Potter and his friends consciously work towards becoming animagi, to keep Remus Lupin company. While someone says this is extremely unusual, the evidence is against that. I mean, take any three teenage wizards. All three try to become animagi. All three succeed. Random? Aye, right.
Far be it from me to over-analyse, of course :-)
Oh, here's The Harry Potter Project