*H₂n̥gʷʰis

*H₂n̥gʷʰis is a reconstructed Proto-Indo-European term meaning 'serpent', as well as a possible name for a mythological entity, polycephalous sea serpent or dragon which was slain by a hero named Trito with the help of the god Perkʷunos.

Evidence
A unifying characteristic of most Indo-European descendant mythologies is a story about a battle between a god of thunder, the great hero and a huge serpentine creature. Indo-Iranian and probably also Hittite traditions use the same Proto-Indo-European root *h₂engʷʰ-, whence *h₂n̥gʷʰis, to denote the serpent.

Hittite mythology
Possible Hittite cognate is Illuyanka, a dragon from a myth known from tablets found at Çorum-Boğazköy, the former Hittite capital Hattusa, and is also found in Catalogue des Textes Hittites. According to the narrative, a thunder god Tarḫunna slays Illuyanka. The contest is a ritual of the Hattian spring festival of Puruli. According to Katz (1998), Illuyanka's name is probably a compound, consisting of two words for "snake", Proto-Indo-European *h₁illu- and *h₂engʷʰ-. The same compound members, inverted, appear in Latin anguilla "eel". The *h₁illu- word is cognate to English eel, and *h₂engʷʰ- is related to Sanskrit ahi and Avestan aži.

Indo-Iranian mythologies
In Indo-Iranian traditions name of a snake stems from the same root, which is reconstructible in Proto-Indo-Iranian as *Háǰʰiš, and is a cognate to Old High German unc, Old East Slavic ǫžь and probably, as previously mentioned, Hittite Illuy-anka. In Zoroastrianism and Persian mythology, Pishdadian shah Fereydun (𐬚𐬭𐬀𐬉𐬙𐬀𐬊𐬥𐬀), slays shah Zahhak, who is also known as Zahhak the Snake Shoulder, and is related to aži dahāka, creatures that frequently appear in texts such as Shahnameh or Garshaspnameh. In those texts, they are slain by various heroes such as Garshasp, Rostam or Esfandiyar

In Hindu mythology, the Vedic god Indra slays the multi-headed serpent Vṛtrá, which has been causing a drought by trapping the waters in his mountain lair. In the Vedas, Vṛtrá is frequently called Ahi (Vedic Sanskrit: अहि ), a Sanskrit descendant of Proto-Indo-Iranian *Háǰʰiš, and in turn, of Proto-Indo-European *h₂n̥gʷʰis.

Descendant tree

 * PIE: *h₂engʷʰ- → *h₂n̥gʷʰis, “snake”
 * Proto-Indo-Iranian: *Háǰʰiš, “id.”
 * अहि, epithet of Vṛtrá
 * 𐬀𐬲𐬌 → 𐬀𐬲𐬌⸱𐬛𐬀𐬵𐬁𐬐𐬀
 * (borrowing)
 * (probably) Hittite: Illuyanka
 * (probably) Hittite: Illuyanka

As for other related Indo-European myths, slayed serpents usually bear names etymologically unrelated to *h₂n̥gʷʰis, but they frequently have a same meaning "snake, serpent": In most Slavic tales, the word that descends from Proto-Slavic *zmьjь ("snake, dragon") is frequently used, with examples being Russian and Ukrainian Zmey Gorynych (Змей Горыныч; Змій Горинич), many-headed dragon who was killed by Dobrynya Nikitich, zmȁj in Serbian and zmej in Macedonian fairytales. In Irish mythology, Oilliphéist (ollphéist, ) are the sea monsters of similar type, the most prominent of which was Caoránach, who was expelled by Saint Patrick to Lough Dearg in Donegal, Ulster. The name of similar Albanian mythical creatures Kulshedra (kulshedër; def. kulshedra), who were slain by heroes drangue, derives from Latin , which roughly translates to an " snake". Armenian dragon Vishap, who was slain by Vahagn, derives its name from an Iranian language, and can also mean "leviathan", "whale", or "Draco".