Snape's Patronus

Eeyore has a great post up speculating about what creature Snape's patronus might be. As I've said before, I like these kind of speculations because regardless of whether they turn out to be correct or not, they are taken directly from the texts (i.e., what we call canon) and they provide insights into characters. Here's an excerpt, comparing Snape to the thestrals: Snape has always seemed sinister to Harry and to the other students. Yet, he has not (until the end of HBP) done anything to harm anyone (as Quirrell told Harry in Philosopher's Stone--Snape hated him (Harry), but he never wanted to kill him), only to save them; he tries very hard to teach them things that will be useful. His talents are likely under-used; he'd prefer teaching DADA because he is an expert and has been since childhood. But he's also quite good at Potion making, and Dumbledore has chosen to have him remain in that position--just as the thestrals are used for transportation and mundane things, rather than being used for their full potential. Earlier, we had a good discussion here and here on Snape's affinity with spiders.