User blog comment:Plexioth- xD/Monster Hunter 3 Ultimate/@comment-58.168.27.3-20121219232250

http://www.capcom-unity.com/dubindoh/blog/2012/12/19/the-monsters-of-monster-hunter-3-ultimate-arzuros

Unless Yuri made a mistake, it seems like they're considering Pelagus and Fanged Beast to be different classes. So Arzuros, Lagombi and Rangurotora aren't in the Pelagus class.