Eeeet's aliiiiiive! AAAALLLLLIIIIIIIVE! **Overly dramatic thunder claps and other cheap fifties special effects.**
Ahem.
I'll try to formulate a reply to your very valid point, but it's still sort of beginning to evolve inside my head, so it probably won't be all that coherent.
It's a name site that gives personality analyses with every name. And the creepy thing is that many of them fit quite neatly. So, if your Animagus form is dictated by your personality, and your personality is defined in some strange and mystical fashion by your name....
As for Rita Skeeter.... The world may never know. Most probably, she became an Animagus to help her continue to be a nasty snitch, and the fact that her Animagus form turned out to be a beetle just made the deal even better.