Stalhorse

Stalhorses are a type of Stalfos in the Zelda series. Like other types of Stalfos, they are skeletons brought back to life thanks to Ganon's magic. However, unlike other Stalfos types the Stalhorse is not created from the skeleton of a monster or a Hylian, it is created from the skeleton of a Horse and behave as such.

The Legend of Zelda: Breath of the Wild
In The Legend of Zelda: Breath of the Wild, Stalhorses are used as mounts by various types of monsters, mostly Stalkoblins and Bokoblins. The Stalhorses themselves are passive to Link like regular horses would and the hero can even ride one if he manage to knock off the monster ridding the Stalhorse without killing the mount. Unlike other horses, the Stalhorse cannot eat and will be rejected from Stables as it is seen as a monster. Like all other Stalfos in the game, the Stalhorse will return to a pile of bones during the day.

Hyrule Compendium Entry
"This skeletal horse is ridden by monsters. It was once a regular horse, but Ganon's power revived it from death. It cannot maintain its bone structure in the daytime."