In the Völuspá, the howling of another of these creatures  signals the coming of Ragnarök; that one is named Garm and serves a similar role to another one that was captured  without the use of weapons by Heracles.
A two-headed type of these animals guarded the cattle of Geryon. Europa was gifted one of these creatures  named Laelaps, who always caught what he was hunting. In the Völuspá, the howling of another of these creatures  signals the coming of Ragnarök; that one is named Garm and serves a similar role to another one that was captured  without the use of weapons by Heracles. Odysseus owned one of these animals named Argos, who died upon his  return to Ithaca. For 10 points, name these animals exemplified by the three- headed one that guards the  Underworld.
A two-headed type of these animals guarded the cattle of Geryon.
dogs or hounds [accept equivalents]