Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† octōgōnŏs (octāgōnŏs), i, adj., = ὀκτώ-γωνία, eight-cornered, octagonal: turris marmorea octogonos, Vitr. 1, 6, 4.
As subst.: octōgōnum, i, n., an octagon, Vitr. 1, 6, 4 al.