In both forms he looks like a behemoth humanoid lion that wields a pair of giant swords. When in this form, his mane and fur turn white, he has red eyes and his armor appears made of silver, probably to look more like a lynx; in his tame form, he has yellow fur and his eyes appear normal.
The source of his evil power and the main reason of The Island's Demise is an even more powerful foe. It is known that in the past a man named Zelham hated him because he got the love and admiration from the island monsters so he made a machine to try and gain control of all the inhabitants of the island. The resul was Zelham's demise and Leonyx having his heart separated from his body in an attempt to control the machine. This made him turn into this form.
This boss can attack twice per turn and three times when he's at the verge of defeat. It has the most powerful magic, breath attacks and strength so far. Its most deadly attack is his lightning storm.
The only form that can be synthesized in DQMJ2, is Leonyx. It requires a Wildcard, a Marquis de Léon, an Orochi and a Don Mole. This evil form is not obtainable in any way in this game, it is however in the Professional edition (only released in japan).