Faerie Folk

In folklore, the NightElf causes nightmares, and can leave one feeling exhausted after waking. This particular kind though is a being who can manipulate dreams to their own whim, and this includes the ability to cause dreadful or delightful dreams alike, depending on their mood or liking for the sleeper in question.
They are shapechangers and can appear as cats, horses, bats, mice, or feathers, for instance.