Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Rhēsus, i, m., = Ῥῆσος, the son of a Muse, a king in Thrace, who was robbed of his horses and killed by Diomede and Ulysses before Troy, Cic. N. D. 3, 18, 45; Verg. A. 1, 469; Ov. M. 13, 249 sq. al.