Hippopotamus

#The roots

• ἵππος (híppos) — “horse”
• ποταμός (potamós) — “river”
• Put them together and you get, literally, a “river horse.”

#From “horse of the river” to one word

• Classical Greek kept it as two words: the adjective phrase ἵππος ποτάμιος (“river horse”) — Herodotus has the plural, οἱ ἵπποι οἱ ποτάμιοι (hoi híppoi hoi potámioi), “the river horses” (Histories 2.71).
• The single-word Greek compound ἱπποπόταμος is post-classical. LSJ’s earliest citations are the Roman-era medical writers — Dioscorides (De materia medica 2.23, 1st c. AD) and Galen (14.241, 2nd c. AD), later Damascius (Life of Isidore 98, early 6th c.); a variant ἱπποποτάμις turns up in a 3rd-c.-AD papyrus (P.Oxy. 1220.21).
• On the Latin side the word runs earlier: the Thesaurus Linguae Latinae records it “inde a Varrone” — from Varro, De lingua Latina 5.78 (1st c. BC) — though in the Greek-looking form hippopotamos/-ios, which Spengel’s edition prints as two words (hippos potamios). Pliny the Elder (Naturalis Historia, 1st c. AD) has it throughout in the -ius form, hippopotamius; the now-standard spelling hippopotamus is later still (Claudian, carm. min. app. 9; glossaries). (TLL VI 2817.)
• English took the one-word form from Latin, in the mid-16th century.

#Relatives that share a root

From híppos (“horse”):

• hippodrome — híppos + drómos (“course, racetrack”)
• hippocampus — híppos + kámpos (“sea monster”); the seahorse, and the brain structure shaped like one
• Philip — Phílippos, phílos (“loving”) + híppos: “lover of horses”
• Deeper still: híppos and Latin equus both descend from the same Proto-Indo-European root (*h₁éḱwos) — so hippo- is a distant cousin of equine and equestrian.

From potamós (“river”):

• Mesopotamia — mésos (“middle”) + potamós: the land “between the rivers”
• Potamogeton — the pondweed genus, roughly “river neighbour”
• potamology — the study of rivers (rare, but it exists)

#The same animal, other tongues

Plenty of languages skipped the Greek loanword and coined their own name — and a surprising number landed on some version of “river horse”:

• “River horse”: German Flusspferd (Fluss “river” + Pferd “horse”) · Swedish flodhäst, Danish/Norwegian flodhest · Finnish virtahepo (virta “current” + hepo “steed”) · Chinese 河马 (hémǎ) · Japanese 河馬 (kaba)
• “Nile horse”: German Nilpferd (Nil + Pferd) · Dutch nijlpaard
• “Water horse”: Hungarian víziló (víz “water” + ló “horse”)
• “Sea cow”: Afrikaans seekoei
• Its own word: Swahili kiboko — also the word for a whip, traditionally cut from hippo hide

#A note on the plural

• hippopotamuses — the standard English plural.
• hippopotami — the Latinate one; strictly a mock-Latin form, applying a Latin plural to a word built from Greek roots.