A One Piece / My Little Pony: Friendship is Magic Crossover.
This story, is a re-telling of the G1 MLP pilot episode "Rescue at Midnight Castle" with G4 ponies, several G1 MLP villains not in the original pilot, and Tony Tony Chopper from One Piece replacing Megan's role.
Takes place after the royal wedding, and before Season 3 for MLP
Also takes place after the Straw Hat's Fishman Island adventure, but before reaching Punk Hazard.
(Warning, some light Chopper X Fluttershy crossover shipping)

During one of the Straw Hat Pirate's voyages after the events of Fishman Island, a storm of the New World strikes their ship, The Thousand Sunny, and sweeps Tony Tony Chopper, the crew's reindeer doctor, overboard. He later awakens in the cottage of a yellow winged pony named Fluttershy who rescued him. Chopper discovers he's in a land called Equestria.
Little do any of the know, ancient evils long forgotten over time, threaten to plunge the land into darkness.