Megabeasts are special and very powerful 'boss' creatures who are created during world generation, each associated with a number of spheres. They are named historical figures, and the game will announce their presence by name. They are distinct from, and larger than, semi-megabeasts.
The megabeast species are the dragon, the hydra, the roc and the bronze colossus. Forgotten beasts and titans are procedurally generated megabeasts.