Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. Mĕgărēus (quadrisyl.), a, um, adj. = Μεγαρεῖος, of or belonging to the city of Megara (in Megaris), Megarean (class.): Euclides Socratis discipulus, Megareus, Cic. Ac. 2, 42, 129.
3. Mĕgăreus (trisyl.), i, m., = Μεγαρεύς, a son of Neptune, and father of Hippomenes, Hyg. Fab. 157 and 185; Ov. M. 10, 605.
Hence, Mĕgărēĭus, a, um, adj., of or belonging to Megareus, Megareian (poet.): heros, i. e. Hippomenes, Ov. M. 10, 659.