This mod is part of The Vanilla Experience.
A minimalistic mod which allows zombie horses to spawn with a zombie riding it. Whenever a zombie spawns, it has a chance to be spawned as a rider. You can configure it to only spawn on the surface and if it should burn in daylight.
You may also be interested in Giant Spawn or Skeleton Horse Spawn.
chanceSurfaceZombieHasHorse (default = 0.05, min 0, max 1.0): The chance a zombie that has spawned on the surface is riding a horse.
shouldBurnZombieHorsesInDaylight (default = true): If enabled, burns zombie horses when daylight shines upon them.
onlySpawnZombieHorsesOnSurface (default = true): If enabled, a zombie horse with rider will only spawn on the surface.