LSJ
ἡ, epith. of cows, of uncertain meaning (yearling, fr. ἔνος³, acc. to EM 432.2, Hsch.), used by Hom. only in acc. sg. and pl. ; gen. ἤνιος A.R. 4.174; — βοῦς… ἤνις ἠκέστας Il. 6.94, 275, 309; βοῦν ἤνιν εὐρυμέτωπον ἀδμήτην 10.292, Od. 3.382. (ἦνιν codd. and Ptol.Oroandae ap. Hdn. Gr. 2.71; ἤνιν Tyrannio ibid. ; perh. a stem in ι.)
Liddell-Scott-Jones, Greek-English Lexicon (9th ed., 1940)