Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† attăgēn, ēnis, m. (cf. Schneid. Gr. 2, p. 131; Neue, Formenl. I. pp. 153, 318, 323 sq.) (attăgēna, ae, f., Mart. 2, 37, 3, and 13, 61), = ἀτταγήν, a meadow-bird, the hazelhen or heath-cock: Tetrao bonasia, Linn.: Attagen maxume Ionius celeber, Plin. 10, 48, 68, § 133: Non attagen Ionicus Jucundior, Hor. Epod. 2, 54.