Requires the library mod Collective.

This mod is part of The Vanilla Experience modpack and Serilum's Spawn Bundle mod.
Zombie Horse Spawn is 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.

Configurable: ( how do I configure? )
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.

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You may freely use this mod in any modpack, as long as the download remains hosted within the CurseForge ecosystem.

Serilum.com contains an overview and more information on all mods available.

Comments are disabled as I'm unable to keep track of all the separate pages on each mod.
For issues, ideas, suggestions or anything else there is the Github repo. Thanks!