Zombie Horse Spawn mod will add to the game a natural spawn of zombies riding a zombie horse, now this mob can appear instead of a regular zombie with a probability of 5% (1 such mob for 20 normal), in the mod settings you can change the probability of occurrence.
Open the file \ .minecraft \ config \ zombiehs-common.toml (or zombiehs-common.cfg) with notepad:
chanceSurfaceZombieHasHorse (standard = 0.05, minimum 0, maximum 1.0): chance of a zombie appearing on a horse
shouldBurnZombieHorsesInDaylight (default = true): if set to false, the zombies on horses will not burn in the sun.
onlySpawnZombieHorsesOnSurface (default = true): if set to false then zombies on horses will also appear in dungeons.
How to install the mod:
Install all additional mods if specified.
Download mod and copy to .minecraft \ mods
In the launcher, launch the version of the game with forge.