Stalhorse Creature
In-game Description
A skeletal horse often ridden by monsters. These animals, which used to be ordinary horses, are said to be revived in areas where there is a high gloom concentration. They can only maintain their forms in darkness, and they have the remarkable ability to run atop gloom.
Notes
Stalhorses can be found in the Depths and are not aggressive – despite their glowing red eyes and skeletal appearance! They can be mounted by sneaking up on them using crouch (left analogue stick button) and mounting them and then soothing them.
Map(s)
Depths
Interactive Map Locations
19. Hyrule Depths Map
3x Stalhorses can be found wandering near Nisoij Lightroot where you fist enter the Depths.
They can also be found on the surface at night. I found some on Nima Plain.