The Proto-Indo-European root is *peth₁- "to rush, to fly" (not to be confused with *peth₂- "to spread"). The suffixed form *peth₁-rā- became Proto-Germanic *feþrō "feather", Old English feðer, then feather.
The o-grade form *poth₁- plus the Greek suffix -amo- became Greek ποταμός potamos "river" ie, "rushing water". This combined with Greek ἵππος hippos "horse" (from PIE *eḱw-o "horse" as in equine) to form hippopotamos "river horse".
The suffixed form *pet-nā- became Latin penna "feather", and this became Old French penne and English pen, as in the writing instrument, which was made from feathers.