He is simply a lion in a cage - The cage is equipped with wheels, several pieces of weaponry and other equipment. We don't know where he came from, and we don't know where he's going. But he's here. And he made a giant shade run away crying.
Caged Lion recently met with the legendary Lord Deus Ex Machina III, the master of all god-modding. Somehow, he was spared with merely being teleported to the site of the Battle of Conquistadores v. Apprentices of Caiaphus. This is where he made the aforementioned large shade cry.
Having been killed several times, and making several returns, the Caged Lion is one of the most feared BZPB characters in history. He is also the leader of the Caged ones.