Possibly I should have saved that title for some fic later on. Or more probably, someone has already used it.
I'm curious about how being in animagus state affects someone mentally. Sirius says, in PoA:
I could transform in my cell... become a dog. Dementors can't see, you know...." He swallowed. "They feel their way toward people by feeding off their emotions.... They could tell that my feelings were less -- less human, less complex when I was a dog... but they thought, of course, that I was losing my mind like everyone else in there, so it didn't trouble them.
So, one night when they opened my door to bring food, I slipped past them as a dog.... It's so much harder for them to sense animal emotions that they were confused...
This is all about the emotional side of things. And Sirius refers to his emotions as Padfoot as "animal emotions". But clearly he's still sentient.
Rita Skeeter uses her animagus form to covertly gather information, so mentally, she must still be very sharp, even as a beetle. (I wonder what beetle emotions are like.) But is it a different kind of intelligence? How much do those doggy and beetle-y emotions affect their actions?
It seems, not so much from the text, but from a remark that JKR made in an interview, when asked what kind of animal she's be if she were an animagus ("I'd like to be an otter - that's my favourite animal. It would be depressing if I turned out to be a slug or something."), that you don't choose the animal you change into.
I wonder if your possible animagus form and the form of your patronus are related. If I had more time before work, I'd add some stuff about His Dark Materials and daemons. Sadly, I don't.