After George shoots Lennie, Lennie wakes up in Equestria. Celestia and Luna had brought him to their world for a chance at a better life.

Yes, Lennie is- and stays- human.
Note: Romance tag is NOT for Lennie.

Disclaimers:
Not to be confused with Shred_Hexstring's Of Mice and Mares, a ponified version of the novel.
If my depiction of Lennie ever becomes offensive to you, please note that is not and will never be intentional.
Original characters Lennie Small and George Milton copyrighted to John Steinbeck.