potamo-

Also petə-, Proto-Indo-European root meaning "to rush, to fly." 

It forms all or part of: accipiter; appetence; appetite; apterous; apteryx; archaeopteryx; asymptote; centripetal; Coleoptera; compete; competent; eurypterid; feather; helicopter; hippopotamus; Hymenoptera; impetigo; impetuous; impetus; iopterous; Lepidoptera; ornithopter; panache; panne; pen (n.1) "writing implement;" pennon; peripeteia; perpetual; perpetuity; petition; petulance; petulant; pin; pinion; pinnacle; pinnate; pinniped; potamo-; potamology; propitiation; propitious; ptero-; pterodactyl; ptomaine; ptosis; repeat; symptom.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pattram "wing, feather, leaf," patara- "flying, fleeting;" Hittite pittar "wing;" Greek piptein "to fall," potamos "river, rushing water," pteron, pteryx "feather, wing," ptilon "soft feathers, down, plume;" Latin petere "to attack, assail; seek, strive after; ask for, beg; demand, require," penna "feather, wing;" Old Norse fjöðr, Old English feðer "feather;" Old Church Slavonic pero "feather;" Old Welsh eterin "bird."

omnivorous ungulate pachydermatous mammal of Africa, 1560s, from Late Latin hippopotamus, from Greek hippopotamos "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. Translated as river-horse in Holland's Pliny (1601).

Ypotamos comen flyngynge. ... Grete bestes and griselich ["Kyng Alisaunder," c. 1300]

Related: Hippopotamic.