Hounds of Tindalos
The Hounds of Tindalos are monstrous creatures which pursue those who leave their own times. Those who travel too far from their proper time (such as by use of the drug Liao, which allows the mind to wander through time leaving the body behind) will draw the attention of the Hounds. The Hounds will then pursue the person through time, tirelessly, eventually catching up with that person in the present and devouring them.
The Hounds of Tindalos are said to have been created by some nameless, foul deed at the beginning of time. They are derived from the 'angles' of time, while humanity has a "pure" element which derives from the 'curves' of time. The Hounds desire to devour this pure part of humans. Being tied to the angles of time, a Hound of Tindalos pursuing its prey can manifest through any angle -- if its chosen prey is in a round room without angles, the Hound cannot appear.
They are described as looking lean and terrible, embodying all the foulness in the universe.
The Hounds of Tindalos first appeared in the story "The Hounds of Tindalos" by Frank Belknap Long.