The skeleton trap horses are a new mob introduced in 1.9 snapshot 15w38a. From the wiki: When a player approaches to within 10 blocks, the following will happen:
Lightning strikes the horse.
The horse will become skeletal if it isn't already.
The horse becomes tame.
The horse becomes fully grown (if a baby) and the breeding cooldown is reset.
IA skeleton is spawned riding the horse.
If on peaceful, the Skeleton will not spawn, while the Horse will.
The skeleton has damage immunity for 3 seconds.
The skeleton will not despawn since they have the tag PersistenceRequired set to true.
The skeleton will be wearing an iron helmet, unless it randomly spawned with some other headgear.
The skeleton's bow and helmet will be enchanted as if on an enchantment table at level 5–22.
The level is higher depending on regional difficulty; on Easy it will always be a level-5 enchantment.
Three additional skeletal horses with riders will be spawned in the vicinity.
