Minecraft wiki entry for Skeleton_Horse, associated text: The skeleton horses becomes passive and tamed if the player kills the skeleton.
The skeleton wears an iron helmet, unless it randomly spawned with some other headgear.
The skeleton's bow and helmet are enchanted as if on an enchantment table at level 5–22. The exact level depends on regional difficulty; on Easy it is always a level-5 enchantment.
The skeleton trap horse becomes fully grown if it was a baby, and the breeding cooldown is reset.
The skeletons do not despawn if PersistenceRequired is set.
A trap horse may spawn in areas that a regular horse may not, such as the middle of an ocean.
A trap horse can trigger normally in clear weather (and often a player encounters a trap horse after the thunderstorm has cleared).
The skeleton drops more experience orbs.
If the difficulty is set to Peaceful, the skeleton rider despawns but the horse remains.
The skeleton does not shoot any mob that attacks the skeleton horse, whether purposefully or accidently. The skeleton attacks only the player.
An explosion is heard, but nothing gets destroyed.