The Lord of the Mountain, also known as Satori, is a spirit in The Legend of Zelda: Breath of the Wild. It act as a guardian deity for the animals "that make their homes in the forest". The Lord of the Mountain can be found near the top of Satori Mountain and it is said to be the reincarnation of a sage that protects Hyrule.
Link can ride the Lord of the Mountain if he approaches it stealthily, however, it cannot be registered at a Stable. The Lord of the Mountain has a level five stamina that replenishes immediately each time a speed boost is used. When Link dismounts it, the Lord of the Mountain will disappears shortly after.
Hyrule Compendium Entry
This noble creature watches over all animals that make their homes in the forest. Legends say this holy creature is a reincarnation of a sage that died on the lands it now protects. It has an acute awareness of its surroundings, so it seldom appears before people. It's sometimes known by its other name, Satori.