[2] Epicyon haydeni is the largest known canid of all time, with the type species reaching 2.4 m (7.9 ft) in length, 90 cm (35 in) in shoulder height and approximately 100–125 kg (220–276 lb) in body mass.

[6] Epicyon had a massive head and powerful jaws that were well adapted for bone-crushing, with enlarged fourth premolars like some hyenas, giving its skull a lion-like shape rather than having a skull similar in shape to that of a wolf; the adaptation would have allowed Epicyon to scavenge as well as hunt, giving it access to the nutritious marrow other contemporary carnivores couldn't access.

[4] Epicyon was one of the last of the borophagines, and shared its North American habitat with several other canids, including: Fossil specimens range from Florida to California and have been found in Nebraska, Montana, Kansas, Texas, New Mexico, Colorado, Oklahoma, Idaho, Oregon, Arizona within the United States, as well as in Alberta, Canada.

[9][10] They are also believed to be social hunters since Epicyon hayden is very prevalent in the fossil record as one of the most common meat-eaters in North America during the late Miocene Epoch period.

[9] Epicyon haydeni's small clavicle, flexible back, and digitigrade posture are all postcranial features shared with other canids and are likely adaptations designed to increase the animal's stride length.

In North America, in places such as Coffee Ranch in Texas, Epicyon shared territory with the bear Agriotherium and the feliform Barbourofelis, machairodont feline Amphimachairodus coloradensis, and fellow canid Borophagus.

Prey for Epicyon included herbivores such as the camel Aepycamelus, the pronghorn Cosoryx, horses such as Neohipparion and Nannippus, the peccary Prosthennops, and the rhinoceroses such as Teleoceras, all of which could provide a suitable meal through hunting or scavenging.