Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
2. Ărī̆us (Arr-), i, m., = Ἀρεῖος or Ἄριος, a renowned heretic, also Ārĭus, Prud. Psych. 794.
Hence, Ărīānus, a, um, adj., pertaining to Arius, Arian, Hier. adv. Lucif. 7.—Ărīāni, ōrum, m., the followers of Arius, the Arians, Hier. adv. Lucif. 7; Aug. Haeres. 49.