Skeletal horses spawned by SkeletonTrap horses allow interaction with their inventory and allow to be saddled, but do not allow to be controlled by a player.

To reproduce:
Summon a Skeletal Horse using
"/summon EntityHorse ~ ~ ~

".
Kill the riding skeleton and sit on the horse.
Open the inventory. Expected behaviour would be to open the survival/cretive player inventory
Further:
Put a saddle in the saddleslot of the horse.
Try to control the horse.