Later in the game, prizes will be given out by the Commissioner's employees. These monsters will be given after you obtain the required amount of skills/monsters.

Effect	How to unlock
Empyrea	Obtain all monsters
Grandpa Slime	Obtain 200 monsters
Leopold	Obtain all skills
Liquid Metal Slime	Obtain 100 monsters
Metal Kaiser Slime	Obtain 150 skills
Metal King Slime	Obtain 100 skills
Robbin' Hood	Complete both monster and skill libraries.