Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Bactrus (-ŏs), i, m., = Βάκτρος, a river near Bactra, now Dehas, Curt. 7, 4, 31; Luc. 3, 267; cf. Isid. Orig. 13, 21, 14; 14, 3, 30; 15, 1, 11; 9, 2, 43.