King Drool (King Tamagodon in the Japanese version) is the main villain of Bonk's Adventure. He is a large green tyrannosaur/dragon-type monster who is intent on taking over the world via enslaving other dinosaurs using mind-control helmets that only Bonk can destroy via his seemingly indestructible head. In the sequels, his grandsom King Drool III takes his place as the main villain.
In the original game (Bonk's Adventure) King Drool kidnapped Princess Za and it fell to Bonk to rescue her from the evil king via fighting his way across the world and defeating his mind-controlled minions in the process.
In the second game (Bonk's Revenge) King Drool III literally steals half of the moon intending to build his own Monster Kingdom, causing Bonk to once again go after him to save the world. Though he is defeated, he would return again in the third game, chaining the half of the moon and attempting to brainwash everyone again.
In Super Bonk, King Drool III somehow obtains a time-travel machine, taking both Bonk and himself to the future, where Bonk is once again tasked with defeating him. After King Drool is defeated, Bonk uses the time machine to take both of them back to their time.
In the last game (Super Genjin 2), King Drool III is imprisoned into his own castle by the Chikuns, who take over his machinations to conquer the land. After Bonk defeats them and releases King Drool, he quickly turns on him, forcing Bonk to fight him one last time.