User blog comment:Aandrew07/Discussion of the Week: Leviathans/@comment-5043481-20130216203856/@comment-4212769-20130217120148

They are classified as Elder Dragons because they are said to summon catastrophes and weather phenomenons. Ceadeus causes earthquakes, Jhen Mohran summons fierce sandstorms. You get the picture.

Also Amatsumagatsuchi and Shantiyen are also Leviathans but put in the Elder Dragon class. Amatsumagatsuchi with the same reason, about Shantiyen not really known as of now.