Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Pŏlybĭus, ii, m., = Πολύβιος, a celebrated Greek historian from Megalopolis, in Arcadia, the son of Lycortas and friend of the younger Scipio Africanus, Cic. Rep. 1, 21, 34; 2, 14, 27; 4, 3, 3; id. Off. 3, 32, 113; Liv. 30, 45.
Pŏlybus, i, m., = Πόλυβος.