Hyaenodon (Hyaena tooth) is an extinct genus of Hyaenodonts, a group of carnivorous creodonts of the family Hyaenodontidae endemic to all continents,
all except South America, Australia and Antarctica, which first evolved 42 million years ago and then suddenly became extinct 15.9 million years ago, existing for approximately 26.1 million years.
The average weight of adult or sub-adult H. horridus, the largest North American species, is estimated to be about 40 kg (88 lb) and may not have exceeded 60 kg (130 lb). H. gigas, the largest Hyaenodon species was much larger, being 120 kg (265 lb) and around 2.5 m (8 ft) long including the tail. H. crucians from the early Oligocene of North America is estimated to be only 10–25 kg ( 22-55 lb). H. microdon and H. mustelinus from the Late Eocene of North America were even smaller and probably weighed about 5 kg (11 lb).
They are fast and vicious dogs with jaws full of teeth, massive heads and short brains. Despite the name, there is no relation what so ever between Hyaenodons and Hyaenas, or even dogs although they share a common ancestor with these carnivoras. Other predators that these carnivores competed food with were the Entelodonts. By 1993, 42 species had been distinguished.