When making an addon, I found an annoying issue relating to mobs. When using minecraft:addrider to add a mob on a horse, certain mobs will not spawn on the horse even when there is nothing stopping them from doing so. This applies to wither skeletons, pigmen, and possibly some other mobs in the game.

Common mobs such as Skeletons and Zombies work fine for minecraft:addrider

I used the correct definitions and names to spawn them, yet the only indication they existed was a single sound they play. It seems the game removes them immediately when they spawn, however I am in normal difficulty. This should not happen.

As seen in this image, I set up Skeleton Horses to have Zombie Pigmen always ride them. Everything is correctly set up, as using zombies works fine.

(It seems that all equine mobs are affected by this regardless of variation. I have no idea if pigs and other ridable mobs are affected by this too. This needs more testing.)