The Hounds of Tindalos

Frank Belknap Long wrote a number of early Cthulhu Mythos stories. These included "The Hounds of Tindalos" (the 1st Mythos story written by anyone other than Lovecraft), The Horror from the Hills (which introduced the elephantine Great Old One Chaugnar Faugn to the Mythos) & "The Space-Eaters" (featuring a fictionalized HPL as main character).
The Hounds are Long's most famous 17 fictions. They're a pack of foul, incomprehensibly alien beasts "emerging from strange angles in dim recesses of non-Euclidean space before the dawn of time" to pursue travelers down the corridors of time. They can only enter our reality via angles, where they mangle & exsanguinate their victims, leaving behind only a "peculiar bluish pus or ichor". They're referenced by many later Mythos writers, including Ramsey Campbell, Lin Carter & Brian Lumley. They've inspired a number of metal & electronic music artists, such as Epoch of Unlight, Edith Byron's Group, Beowulf, Fireaxe/Brian Voth & Univers Zero, all of whom have recorded tracks based on the story.