How to Make Skeleton Horses Spawn More Often
Find and open the config file
After launching the game once with the mod installed, a config file is generated in your config folder. Open it with any text editor. Look for the entry named chanceSurfaceSkeletonHasHorse.
Raise the spawn chance
This value is a decimal between 0.0 and 1.0 that represents the probability a newly spawned skeleton will be riding a skeleton horse. The default is a low value, which is why you rarely see them. Increase it toward 1.0 to make almost every skeleton spawn mounted. A moderate bump gives you more encounters without flooding the world.
Save the file and reload the world (or restart the game) so the change takes effect.
Test your setting
If you set the chance high, most skeletons should appear on horseback. If you still see none, double-check you edited the correct line and that no other mod is overriding skeleton spawns.
Keep in mind that a very high chance makes skeleton horses extremely common, which can make the world feel crowded and combat much harder, so tune it to taste.