Huh, I was sure you are able to choose your Animagus form, but it appears I was mistaken.

Apparently you become the animal that suits you best.

Still, there is a potential for a creative Legilemency and False-Memory Charm casted on oneself in order to create an appropriate self-image. Assuming Bellatrix was an Animagus before meeting Voldemort, was her Animagus form changed when she was shattered into pieces and re-combined into someone else?

Also, what if I Memory-Charm myself to believe that common characteristics of spiders are intelligence and courage? Will my Animagus form change depending on the beliefs of native population (e.g. if you are very cunning, you will be snake in Britain, fox in Russia and mongoose in Asia)?

ETA: Can't stop thinking about it. Created a topic on Reddit since I feel like Reddit is more suitable for a this discussion: LW is serious and I prefer it to stay this way.