Mesogog

Mesogog is the primary antagonist in Power Rangers: Dino Thunder.

Creation
Anton Mercer was on the verge of a scientific breakthrough during an experiment involving dinosaur DNA, when he was accidentally transformed into Mesogog, an evil mastermind with plans to revert the earth to the age of dinosaurs and turn the human race into reptilian creatures like himself.

Mesogog and Dr. Mercer have distinct personalities, but share the same body in a Jekyll-Hyde relationship. Each is aware of the other's existence, but also dislikes the situation. In his Mesogog form, he is equipped with cybernetic technology and psionic powers that he uses to torture his enemies, or his allies when he is angry at their failures.

Dr. Anton Mercer has an adopted son, Trent Fernandez, who became the White Dino Ranger when he accidentally discovered Mesogog's lab while investigating his father's mysterious comings and goings.

Mesogog's Grand Plan
In his world domination attempts, Mesogog is assisted by two servants, Elsa, and Zeltrax, a cyborg whom Mesogog created à la Dr. Frankenstein from the ruined body of Tommy Oliver's former associate, Terrence "Smitty" Smith. Mesogog also controls an army of cybernetic dinosaurs called Tyrannodrones which were originally created by Tommy and Anton.

Mesogog, like Vexacus, originally hated Lothor and believed him to be an idiot. Elsa agreed and was quick to refer to Lothor as a buffoon. For some reason, Mesogog believed his first assault on Reefside would be credited to Lothor. It was never revealed if somebody actually suspected Lothor was involved with that. Mesogog eventually overcame his prejudices and teamed-up with Lothor when he discovered that he had "Evil Power Rangers". Mesogog was not fully allied with Lothor, because he did not trust him completely. When Lothor and Mesogog's team-up idea failed, Mesogog defeated Lothor and shrunk him inside of a jar to add to his collection of experiments. (Lothor and his jar were presumably destroyed with Mesogog's lab at the end of the series.) However, Mesogog had previously told Elsa he already intended to get rid of Lothor even if the team-up resulted in a success.

Destruction
Eventually Mesogog freed himself of his humanity by using a potion to separate out Dr. Anton Mercer. As a result, his insanity and determination increased. He also rid himself of Elsa when needing an energy supply to power his cannon. It took a bit of deception from the Rangers (using the Dino Gems as bait) to destroy the cannon and Mesogog's base. He would later reappear after the Zords were destroyed, absorbing enough energy from the Dino Gems to survive the explosion and 'evolve' into a self-replicating dinosaur-like monster, known as the Mesomonster. As the Mesomonster, Mesogog was nearly invincible and had the ability to self-replicate. Eventually, the Dino Rangers had to sacrifice their powers by using all of the energy within the Dino Gems to finally put an end to Mesogog (although they would regain them next season, in the Dino Thunder - SPD teamup).

Dr. Mercer survived the whole ordeal, and was thankful to the Rangers (especially Tommy) at the series' end.

Trivia

 * After sepearting from Anton Mercer, Mesogog's voice loses its lower pitch, becoming more of hissing sound.
 * Mesogog has no Super Sentai counterpart, the main villain of Bakuryu Sentai Abaranger is Creative Messenger Mikela, though he was later used as the character "Izzy" in the episode "Thunder Storm".
 * Mesogog is the only major antagonist in which he knows about the previous antagonist even before meeting him. In this case, Mesogog seems to know Lothor personally.