The headword is attested in this form (dative singular) in Lexicon Gudianum and the derivative entry of [Zonaras] 799.27. Adler also cites Lexicon Ambrosianum 1465. The genitive form ἐπήτυος occurs in Homer, Odyssey 21.306 (Web address 1), and all other attestations of all forms of the word seem to derive from commentary on this passage.
For apparently related words see epsilon 2216, epsilon 2217.