User blog comment:Flamebiplane/Gahillo Family/Wavedraw Combat Elite Q&A/@comment-30014014-20190717134640/@comment-28718853-20190802145215

"Siege Onis are primarily Ushi-Onis that are made giant with certain gear and given siege cannons and platforms for other mamono. Sometimes unaffiliated mamono will hitch a ride to help in invading whatever town or city we're sieging," Mythria explains.