Was about one metre at shoulder height and 2.25 metres long (7 ft 6 in), it was the world's first large browsing mammal (the biggest mammal ever at the time). Coryphodon had a semi-aquatic lifestyle, like a hippopotamus, although it was not related to modern hippos or any other animal known today. It probably lived in swamps and marshes. Coryphodon was widespread in North America between 59 and 51 million years ago. It is regarded as the ancestor of the genus Hypercoryphodon of Mid Eocene Mongolia.
Coryphodon, especially males, had short tusks used to uproot swamp plants. The creature was very slow due to its long upper limbs and short lower limbs, which were needed to support its weight. Coryphodon had the smallest brain/body ratio of all mammals, living or extinct, possessing a brain weighing 90 grams at a body weight of 500 kilograms.