The Baboon King appears in the Old Forest Mine after accepting the quest Rise of the Baboons and defeating the three Raging Baboons. This quest isn't available until approximately halfway through the game. The Baboon King isn't particularly dangerous on his own but he is capable of summoning two Healing Rollers. If they are not killed immediately, the Baboon King will throw them at your party, dealing massive damage and healing himself for hundreds of HP.