Ocerios was once a human king of the kingdom Lothric.  He became fascinated with dragons, and eventually found the writings of Seath the scaleless, a madman who attempted to turn himself into a dragon.  Ocerios also became mad in his obsession, conducting draconic experiments on his people.  Ocerios eventually became just like Seath, a scaleless dragon-humanoid.  He has blue skin, fungus coming out of his back, and is convinced that he has a child named Ocelotte, even though the child isn’t real