How to Restrict Skeleton Horse Spawns to the Surface Only
Understand the default behavior
Skeletons spawn in many places, including caves and other underground areas. Without restrictions, a skeleton deep underground can spawn riding a skeleton horse, which is usually unwanted and surprising.
Enable surface-only spawning
Open the mod's config file. Locate the option named onlySpawnSkeletonHorsesOnSurface. Enable it so that mounted skeletons only appear on the surface. Save the file and reload the world.
What changes in practice
With this enabled, you will only encounter skeleton horses outdoors, typically at night or in shaded surface areas where skeletons can spawn. Underground skeleton spawns will still happen, but they will not be mounted.
This setting pairs well with a higher spawn chance, because you can raise the probability without worrying about caves filling up with skeleton horses. It also makes skeleton horses easier to find, since you know to search the surface rather than exploring deep caves. If you actually want underground skeleton horses for a farm or trap, simply disable this option again.