A Mare is a Fae that causes its prey to experience nightmares.
A Mare feeds off the resulting fear from nightmares. Mare most often feed on humans, simply because they are considered "fair game" and have little psychic defense against them. Partially for this reason, Mare are almost exclusively Dark Fae.
It has been shown that after the initial feeding, Mare are capable of creating a "psychic connection" with their prey that will allow them to telepathically feed when their prey is sleeping.
A "mare" is a spirit or goblin in Germanic folklore which rides on people's chests while they sleep, bringing on bad dreams (or "nightmares"). The mare is attested as early as in the Norse Ynglinga saga from the 13th century, but the belief itself is likely to be considerably older. The mare is often similar to the mythical creatures succubus and incubus, and was likely inspired by sleep paralysis.