One Piece: Chopper no Daibouken

One day, the Straw Hat Pirates travel to a mysterious island. While the Straw Hats explore around the island, they all suddenly turn into animals; Luffy into a monkey, Zoro into a tiger, Sanji into a fox, Nami into a cat, Usopp into a kappa, and Robin into a parrot. Only Chopper remains as he was. It is up to Chopper to not only help the crew to turn back to their original state, but to also figure out the cause of their mysterious transformation.