Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Ăgăthyrsi, ōrum, m., = Ἀγάθυρσοι, a Scythian people(in what is now Transylvania,and the Bannat of Temeswar)who commonly painted their faces and limbs; hence Vergil: pictiAgathyrsi, A. 4, 146; cf. Plin. 4, 12, 26; and Mel. 2, 1.