The Hounds of Tindalos are extraterrestrial entities created by Frank Belknap Long, first appearing in his 1929 Weird Tales story "The Hounds of Tindalos" and later incorporated into the Cthulhu Mythos by August Derleth.
They are said to "inhabit the angles of time, while other beings descend from curves," and can materialize through any sharp corner measuring 120° or less — first as smoke pouring from the corner, then the head, then the body.
Once a human becomes known to a Hound, it will pursue its quarry through anything to reach it; people attract their attention by engaging in time travel. They are said to possess long, hollow tongues or proboscises to drain victims' body-fluids and to excrete a strange blue pus or ichor, their form too foul to ever be described.