Shadow people spawn during nightmares. Up to two of them will spawn at a time. Health: 7.5 Hearts Damage: 3 Hearts This mob is based on the Shadow Person. They are often associated with sleep paralysis. https://en.wikipedia.org/wiki/Shadow_person
This hag can spawn during a nightmare. She will show up in groups of one. Health: 15 Hearts Damage: 2.25 Hearts This mob is based on the night hag, who is frequently associated with sleep paralysis. https://en.wikipedia.org/wiki/Night_hag
Phantom spiders can spawn during nightmares. They spawn in groups of 3-8. Health: 4 Hearts Damage: 1 Heart These are based on normal spiders. Their visuals are based on a render error from when I tried to do random eye colours.