Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Būbōna, ae, f. [bos], the protecting goddess of cattle among the Romans, Aug. Civ. Dei, 4, 34.
Hence, Būbetĭus, a, um, adj., of or for Bubona: ludi, Plin. 18, 3, 3, § 12.