Minecraft wiki entry for Command_blocks_and_functions, associated text: As of 1.12, passengers can control their mounts to an extent, which means that former issues with Zombie passengers are no longer extant. A more in-depth look at the behavior of passengers and mounts can be found at Tutorials/Summoning Jockeys.
The skeleton horse has Tame set to 1 (yes). Note that there is currently an ongoing bug where summoned mobs riding horses with saddles cannot move, regardless of whether the horse is tame. Technically you can have multiple levels of riders, although this gets silly quickly.