imp - a small mischievous being, like a minor goblin. Etymologically the word imp originally meant “shoot or sprig of a plant”, then “child or young”, and then “little demon”, as in “imp of the devil.” They are often the familiars of witches. (northern European)
incubus - a demon who rapes sleeping women, this is, sadly, a monster present in various incarnations in mythologies around the world.
impundulu - a man-sized black and white bird that summons thunder and lightning with its wings. It’s also vampiric, and is known to associate with witches. (South African)
inkling - a small black spiky creature who looks rather like a humanoid inksplotch thrown upon the wall or floor. The inkling can suck up the words out of books, and write words into the real world with its finger, thus transferring magic between the worlds in and outside of stories. As libraries are its favored habitat, this is one you may not be able to avoid by hiding out among the I's. (I made this one up in a story I never finished writing.)
[Pictures: A witch feeding imps, wood block print from an unknown source, 1579 (Image from ExecutedToday);
Inkling, illustration by AEGN.]