A snake ... very stylish but very obvious too.
A raven ...? Maybe - because of his billowing black robes, hooked nose, etc., I think it makes a sense. --> What do you think, Aberforth? Hey, what is your Patronus, anyway??? A goat??? (just joking, no offense, please! And I like your arguments as well as your goats:)
A bat ... Yes, if you believe the theory about Snape being a vampire. I thought this theory was ridiculous, but I have read some cool supportive arguments (fathomless eyes - because vampires have no souls, etc.). But - if Snape is a vampire, does he need a patronus at all??? We don't know it yet. Can't wait to see ...!
And about childhood - I think JKR gives it major importance - Voldie an orphelin, Snape - violence in the family, Sirius - arguments with perents, ...
Maybe you think I shouldn't have put Sirius in such a company but although he's on the good side he was disagreeable and impulsive, always tried to argue, and when I read his scenes, I always thought he really should cool down.