Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† ellĕbŏrus (hell-), i, m., and ellĕ-bŏrum (hell-), i, n., hellebore, in pure Lat. veratrum, a plant much used by the ancients as a remedy for mental diseases, epilepsy, etc.; the best grew on the island of Anticyra, in the Aegean Sea, Pl. 25, 5, 21; id. 25, 13, 94; Gell. 17, 15; Plaut. Ps. 4, 7, 89; Verg. G. 3, 451; Hor. Ep. 2, 2, 137; Cels. 2, 13; Sen. Ep. 83, 25.
† hellĕbŏrus, etc., v. elleborus, etc.