Skeleton Horseman

The Skeleton Horseman is a Jockey-type mob that was introduced in The Combat Update. A skeleton trap horse can spawn during a thunderstorm, an uncommon type of weather in Minecraft.

Mechanics
Skeleton horseman can spawn on any level of difficulty, but they have a very small chance to spawn if the game is set to "Easy." The lightning is triggered when a player moves within 10 blocks of a skeleton horse, and upon being approached, the horse transforms into a skeletal form. A skeleton rider wearing enchanted gear will also appear atop the skeletal horse. In addition, three more skeletal horses with riders will spawn in the vicinity.

Skeleton horse riders will always spawn with an enchanted iron helmet and a bow which can also be enchanted. They are immune to damage for the first three seconds after spawning. This type of mob follows nearly the same attack strategy as a normal skeleton except it moves much faster due to the added maneuverability of its horse.

Skeleton Horsemen can spawn in diffrent variants. Strays or Wither Skeletons.

Trivia

 * When in a minecart, a skeleton trap horse will be damaged which will result in only a skeleton.
 * Skeleton horsemen are one of two mobs added in the 1.9 update, along with shulkers.
 * Skeleton horsemen can be spawned on top of other horses through commands or external editors.
 * If the only the skeleton is killed, a player will be able to ride the horse since it spawns preset as tamed.
 * In one of the 1.9 snapshots, skeleton trap horses dropped Ender crystals as a placeholder for their actual drop.
 * In the Windows 10 edition 0.15.10, the skeleton horse has a spawn egg.
 * Before 1.9, skeleton horsmen could only be spawned through commands; however, it would only be a regular skeleton horse with no rider.
 * If the mob is name tagged, regardless of where the cursor is, it will still show up on the skeleton.
 * This also means that if it were named Dinnerbone or Grumm, only the skeleton rider would turn upside down, but it would leave the horse right side up.
 * There is currently a glitch where the skeleton may dismount from the horse, but it will float in the air and cannot be attacked. Fortunately, it does not attack players when this glitch occurs.
 * When in peaceful mode, the horse will change. However, the skeleton rider does not appear since it is a hostile mob.
 * When a new skeleton trap horse group spawns, a strike of lightning will occur.
 * It is possible to get a wither skeleton rider instead of a normal skeleton rider if the skeleton trap horse is summoned in the Nether with commands.
 * A player can use skeleton trap horses to their advantage by farming the skeleton trap horses for enchanted iron helmets while also gaining experience, arrows, and bones.
 * If triggered on water, the skeletons and horses will sink into the water, causing the skeletons to eventually take drown damage if they don't have a helmet with Respiration. It is also likely that the horses will drown.
 * You can spawn skeleton horseman by using this command: /summon skeleton_horse ~ ~ ~ {SkeletonTrap:1} (1.11 and newer versions)