One of the unanswered questions is just what would Severus have for a Patronus. In answering a question about Patronuses, Rowling told us that each person's Patronus is unique, just as each of us is unique, and that the person doesn't choose which Patronus he or she has.
The subject of what Patronus Snape would have is a long one over at the Leaky Lounge. The following is my post in the discussion.
In re-reading part of Order of the Phoenix, I was struck by the description of thestrals. Even though we saw thestrals, along with Harry, when they returned to Hogwarts, we first learn what they are in Chapter 21, "The Eye of the Snake", when Hagrid takes the COMC into the forest for a lesson.
But I found it useful to look at the different times we saw thestrals, and their description. Harry first saw them pulling the carriages when they returned to Hogwarts, but didn't know what they were. He's quite surprised to learn that he could see something that Ron and apparently most of the others, couldn't--except Luna.
OP, p. 196-7, US version:
There were creatures standing between the carriage shafts; if he had to give them a name, he supposed he would have called them horses, though there was something reptilian about them, too. They were completely fleshless, their black coats clinging to their skeletons, of which every bone was visible. Their heads were dragonish, and their pupil-less eyes white and satring. Wings sprouted from each wither--vast, black leathery wings that looked as though they ought to belong to giant bats. Standing still and quiet in the gathering gloom, the creatures looked eerie and sinsiter.And on page 199, when Luna says she can see them too, and that Harry is not going mad, he says:
"Can you?" said Harry desperately, turning to Luna. He could see the bat-winged horses reflected in her wide, silvery eyes."
We've heard Snape described as bat-like often enough in the books that quite a few people thought he must be a vampire bat, until Jo shot down that theory. But with all the swooping about that he does, and the sinister manner he has, he is very much like the thestrals--giving the first impression of something mysterious and eerie.
The next time Harry sees a thestral, it is flying over the Forbidden Forest--he has gone to the Owlery to send Sirius a letter, and as he looks out towards the Forest he sees it(OP, p. 282):
A great, reptilian winged horse, just like the ones pulling the Hogwarts carriages, with leathery black wings spread wide like a pterodactyl's, rose up out of the trees like a grotesque, giant bird. It soard in a great circle and then plunged once more into the trees."
But as Cho then enters, that's as much attention as Harry pays to the strange winged horse that he just saw.
(OP, p. 444, US)
When Hagrid takes them into the forest for a lesson, Harry sees them again:
A pair of blank, white, shining eyes were growing larger through the gloom and a moment later the dragonish face, neck, and then skeletal body of a great, black-winged horse emerged from the darkness. It surveyed the class for a few seconds, swishing its long black tail, then bowed its head . . .
Snape has black eyes, rather than white, but his eyes are closed and expressionless and unreadable to Harry, just as the thestral's eyes give no clue to its thoughts or intentions. Snape very often observes what Harry and the others are doing--sometimes commenting, sometimes not.
(OP, p. 446, US):
Hagrid tells the class that Hogwarts has a whole herd of them.Parvati says they are "really, really unlucky! --They're supposed to bring all sorts of horrible misfortune on people who see them."
Hagrid assures the class that that's just superstition--
"they aren' unlucky, they're dead clever an' useful! 'Course, this lot don' get lot o' work, it's mainly jus' pullin' the school carriages unless Dumbledore's takin' a long journey an' don' want ter Apparate--"
Hagrid tells them that the thestrals won't hurt them, and then talks about who can see them and who can't. (Only people who have seen death can see them, which explains why Harry, Neville, and Luna can see them, and most of the rest of the class cannot.)
After a frustrating interruption from Umbridge, Hagrid tells the class more about thestrals.
(OP, p. 448-9, US)
"Well, once they're tamed, like this lot, yeh'll never be lost again. 'Mazin' senses o' direction, jus' tell 'em where yeh want ter go--"
While all this is a great set-up for Harry's later need of quick reliable transportation to London, the description kept making me think that it was also a great metaphor for Snape.
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.
Dumbledore sometimes uses a thestral for a trip to London when he doesn't want to Apparate--that makes me wonder what he has Professor Snape doing other than teaching Potions. Oh, wait--we know that one. He's turned spy for the Order of the Phoenix, often at great personal risk.
If the Patronus gives us clues about what the person's true character and nature are, then a thestral seems a strong possibility for Snape, who remains an enigma to the students and to the rest of the teachers. His appearance isn't warm and cuddly and never will be; he has an air of danger about him, just as the thestrals seem menacing, even though they make no move to attack any of the students. The thestrals can be relied on to take a person where they need to go, with very little or no direction needed. Severus Snape, as a spy, has been in much the same postion--he more than likely would need to rely only on himself in spying on Voldemort and the Death Eaters, all the while needing to have a clear idea of his destination.
Even the very last time we see Severus, he is continuing to do the task Dumbledore set for him--protect Harry (and the other students), and guide and teach Harry so that he is prepared to face Voldemort. On Dumbledore's word we accepted, as they did, that Snape was highly useful--guiding the Order in the fight against Voldemort--never losing his way, as it were.
I do like the idea of Snape's Patronus being a gryffin (see the comments for the post on Harry Potter and the Deathly Hallows--the title for book 7), because of the Christ symbol, but I think it's possible that Rowling is using Snape's Patronus to give us a clearer picture of his character--and we may find those Christ-like connections in his actions rather than his Patronus.