User blog comment:BannedLagiacrus/Temnoceran Discussion/@comment-27125903-20161112070016/@comment-6050823-20161112145851

Really Capcom never specified what characteristics make up the Temnoceran, Snake Wyvern, and Amphibian classes (Technically, Fanged Wyvern class as well but, MH3G Guide Book gave the class a short simple description) so it is really up to them what appears in that class and what characterizes it. In Nerscylla's case, it was its extendable mouthparts that made the Guild put it in its own class.