We never added horses to the list of creatures that can spawn in our biomes.

Presently horses don't spawn in any of our biomes.

We will be adding the chance for horses to spawn in the following biomes:

Meadow - 10%, spawn rate of 2-6
Savanna - 5%, spawn rate of 2-4
Mountain Ridge (Red Rock) 5%, spawn rate of 2-4
Shrubland - 10%, spawn rate of 2-6

Default per vanilla biome is 5%, with a spawn rate of 2-6.