Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Thestĭus, ii, m., = Θέστιος, a king of Ætolia, father of Leda and Althæa, and of Plexippus and Toxeus, Ov. M. 8, 487; Hyg. Fab. 77; 155; 174.
Hence,