Is the rabbit real?

Whether the rabbit is real is irrelevant because its appearance is obvious allegory at the conclusion of an obviously satirical story, shortly after the reference to Animal Farm is made explicit:

Earlier in the movie Crystal told her twisted version of "the box turtle and the jackrabbit" fable in which, even though the turtle beats the rabbit in the race, the rabbit wins in the end by killing the turtle's whole family and eating their dinner. "The jackrabbit always wins."

At the moment the jackrabbit appears we, like Crystal, believe that she is dying even though she won the race (figuratively) and killed all of her antagonists (literally). The allusion to the fable is unambiguous.

For supporting reference: There is an earlier point in the movie in which Orwell walks out of the forest and Crystal asks Don, "Do you see a pig wearing a shirt?" The incident, and her line, is just funny (... as is what they do with the pig later). There might be some significance to the fact that Crystal is explicitly questioning what she is seeing (which isn't a stretch given her recent exposition that she had been knocked out by drugs for roughly 18 hours), but it's otherwise consistent with the deadpan humor throughout the film.