A little while back, I was skimming through all the parts in OotP for Neville parts because I was looking for something... I forget what though... but it wasn't Neville relate... oh wait, yes it was. I was looking to see if he ever conjoured a patronus, which he never did.
Harry says to him "You've got to think of something happy," and Neville responds miserably and sweating with his efforts "I'm trying."
That made me sad that he hasn't got much to be happy for. He's got a miserable home life if you think about it. His family is not very proud of him, and spent most of his childhood trying to frighten the magic out of him. And they treated him like he would never have magic, and made him incredibly inconfident in himself. His Gran basically tortures him emotionally without even knowing she does it.
But as I was saying, I think his patronus might be a woodpecker because it symbolizes prophecy, magic, power, guardian of trees, and it heralds rain and storms.
Obviously, the prophecy could have referred to him, he's magical, he's got a lot of power that he has finally learned how to use, he's great in herbology, and england, especially northern england where it is presumed he lives is very rainy.
Here's some other's I saw that I liked for him, but woodpecker fit him best.
Buffalo - Sacredness, life builder. The buffalo provides all good things for living, and bestows great curing powers.
Frog - Power of water, sensitivity, medicine, hidden beauty, power.
Eagle - Divine spirit, air, the sun, power in battle, protection from evil, clear vision, success, prosperity, wealth, intelligence, renewal, courage.
Spider - Creative, pattern of life, connects the past with the future, creating possibilities.
Woodpecker - Prophecy, magic, power, guardian of trees, Heralds rain and storms.
I just searched for animals and their symbolism and meanings and got a good thing that had a bunch of things like that.
Here's a website to some animal symbolism:
http://www.princetonol.com/groups/iad/lessons/middle/animals2.htmIf you want to look up more animal symbols, try googleing "Celtic Animal symbols" (mostly because the books take place in England). If you want to find animals for yourself, just search animal symbols or look at that site.
And for those who think a wood pecker is small, I have a strong feeling that if a Patronus is small, it enlargens... because Hermione's patronus is an otter... That is a small creature, and a very cute one at that.