What should Neville Longbottom's Patronus be?

Oh my goodness! I am doing a fanfic featuring Luna and Neville. I know Luna's Patronus is a rabbit, but the books don't mention Neville's Patronus. I was thinking his should be something that when it's born it is very weak and wimpy, but as it grows, it gets stronger until it can do something great. Or, at first glance, you take it for granted and think that it can't hurt you, but then-- suprise, surprise, surprise-- you're dead. Would a hippopotamus (hippo) be good? but i want it to be able to cuddle with a rabbit...