Desmostylus[1] is an extinct genus of herbivorous mammal of the family Desmostylidae living from the Chattian stage of the Late Oligocene subepoch through the Late Miocene subepoch (28.4 mya–7.250 Mya) and in existence for approximately 21.2 million years.

[2] Desmostylus was a large, hippopotamus-like creature, with the adult Keton specimen of D. hesperus measuring 275 cm (9.02 ft) in length, 105 cm (3.44 ft) in height and 1,283 kilograms (2,829 lb) in body mass; the largest known humerus, which is 1.3 times that of the Keton specimen in length, probably belonged to an individual with a mass of 2.8 metric tons (3.1 short tons).

[4] Most likely fully aquatic, Desmostylus is thought to have lived in shallow water in coastal regions, usually less than 30 meters deep.

[6] Its less dense bone structure suggests that Desmostylus had a lifestyle of active swimming and possibly feeding at the surface, unlike other desmostylians that were primarily slow swimmers and/or bottom walkers and sea grass feeders.

[7] Desmostylus hesperus (synonyms and invalid names: D. watasei, D. cymatias, D. californicus, D. mirabilis, D. minor, Desmostylella typica),[2] D. coalingensis (syn.