What kind of horse only comes out of night?

The phrase "a horse that only comes out of the night" is often associated with the mythical creature known as the "Nightmare," a type of horse from folklore that is said to appear in dreams or during the night. In a more literal sense, it could refer to nocturnal horses or those that roam under the cover of darkness for safety or feeding. Additionally, it could symbolize a metaphorical concept, representing hidden fears or desires that emerge in the dark.