The island is getting full of factions: Luffy's bunch, the centaurs (plus harpy), the fog dude with the gas mask troops, the samurai, the kids, and now the marines as well.

Maybe the centaurs were made by the same person who sliced the samurai. After all, the samurai's parts can attach elsewhere and stay alive, so it's possible the centaurs are just regular humans who got their upper body attached to an animal's body (minus the animal's head).