Ikelos

Ikelos who was one of the Oneroi was the personifications of nightmares and appeared in various types of dreams.  He made shapes of humans appear in dreams.

Ikelos was a son of Nyx. He resided with his brothers, Morpheus and Phantasos in the Underworld. His appearance was that of a daemon, but he appeared in the form of animals and monsters when entering dreams, most commonly a lion or bear.