Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
3. ăcus, i, m. [1. acus], a kind of sea-fish with a pointed snout, the hornpike or gar-pike (Gr. βελόνη): acus sive belone unus piscium, etc., Plin. 9, 51, 76, § 166: et satius tenues ducere credis acos, Mart. 10, 37, 6; cf. Plin. 32, 11, 53, § 145, where belonae again occurs. (Some read una for unus in the passage from Plin., and acūs for acos in Mart., as if these forms belonged to 1. acus.)