Stalhorse

Stalhorse (also known as Bone Horses) are an undead species born from reanimated Horse skeletons that appears in The Legend of Zelda: Breath of the Wild. They are used as riding mounts by Stalkoblin.

The Legend of Zelda: Breath of the Wild
Stalhorse are a rare Stal variant of Horses that are occasionally used by Stalkoblin as riding mounts. They can be encountered by Link in a few areas of Hyrule at night ridden by Stalkoblin such as Rutile Lake or N. Tabantha Snowfield. Like Bullbos used by Bulblins in Twilight Princess, Stalhorse will not be hostile towards Link when riders are killed, though they will flee from him. However if he can mount them they will automatically be tame and can be ridden just like living horses. However like wild animals such as Bears and Deer, if Link tries to register it, the Stable Clerk will consider it a monster and refuses to register or board it out of fear that it may eat the living horses. It is unknown if Stalhorse actually eat their living counterparts as it may simply be fear and due to associating it with other Stal beasts such as Stalfos, Stalkoblin, Stalmoblin, and Stalizalfos. Other characters will also act fearful (if unarmed) or hostile (if armed) towards Stalhorse if Link rides it near them again suggesting that most people fear it due to its appearance and undead nature, despite its apparent docile behavior when rider-less. Like other Stal enemies encountered outside areas, Stalhorse only appear at night. They will also die when night ends at 5:00 AM (sunrise) meaning Link can only ride them at night (9:00 PM to 5:00 AM).

After obtaining the Camera Rune, Link can take a picture of a Stalhorse to add Stalhorse to the Hyrule Compendium. Stalhorses are often referred to by most Hylians as Bone Horses and are considered by some to be a mythical creature due to their rarity.

Side Quest
There is also a side-quest called Stalhorse: Pictured! in which Juannelle asks him to take a picture of a Stalhorse (which can be found at night at an enemy base north of the Snowfield Stable where Link receives the side quest) and show it to her. She will reward Link with a Silver Rupee (100 Rupees). Link can only receive this quest from Juannelle after obtaining the Camera Rune.