What determines the form?
I'm never actually sure if the whole "protector" thing is an extrapolation, because Luna's Patronus is a rabbit, and Ron's is a terrier. I always kind of thought that it has to represent yourself in some way too. I think that it has to represent you as well. I always thought that Hermione's Patronus was an otter because they are very clever animals, as is she. (And it was JKR's favourite animal.) Ron's is a loyal terrier. So, like they represent some of your qualilities, I suppose. But then again, I suppose people would find comfort in security in their good qualities. Cho's Patronus is a swan, which is often a symbol for beauty. I mean, I know that they can be super visious and all, but I think that again it is representing Cho. It just appears that many people's Patronus represent themselves in some way.