Heh Heh, Benny
Well anyway, like Vosur and WingedZephyr said, its based on what animal you feel closes too.
Sometimes that animal might change ( I know a few in the forums whos animals/Fursonas have changed ). Sometimes it can be a hybrid or whole bunch of animals in one you feel closes too. ^^
I chose a ferret because Im hyper-active, Cuddley, Random, Rare(Personalility wise), Cute, Sometimes Misunderstood, and a few other attributes.