omnivorous ungulate pachydermatous mammal of Africa, 1560s, from Late Latin hippopotamus, from Greek hippopotamus "riverhorse," an irregular formation from earlier ho hippos potamios "the horse of the river"), from hippos "horse" (from PIE root *ekwo- "horse") + adjective from potamos "river, rushing water" (see potamo-). Replaced Middle English ypotame (c. 1300), which is from the same source but deformed in Old French. Glossed in Old English as sæhengest.
Ypotamos comen flyngynge. ... Grete bestes and griselich ["Kyng Alisaunder," c. 1300]