3,000 years ago, the land of Animaria waged a war with an evil who rose from the Earth called Orgs. Six Ancient Warriors defended the land from these creatures. Five of them used the powers of the Crystal Sabers, while the sixth, Merrick, was assigned to protect Princess Shayla, who he also had feelings for. To keep some of Animaria's culture safe, it was lifted into the sky by the Ancient Warriors. On this floating island, a slumbering Shayla and five Power Animals were kept safe. The warriors defeated most of the Orgs, but they could not defeat Master Org, who destroyed the Animus Megazord.
With little hope left, Merrick set out to find a legendary wolf mask which was said to give great powers. He found the mask at Nayzor's temple, along with three Animal Crystals. He put the mask on, and summoned the Wolf, Alligator, and Hammerhead Shark Zords to form the Predazord, which was able to destroy Master Org. The victory came with a price. Merrick was unable to remove the mask, which had corrupted him, made him evil. As the evil Zen-Aku, Merrick was out of control, even attacking the Ancient Warriors. The warriors had no choice but to take Zen-Aku down, and imprison him in a tomb.
In present day, the new Master Org wages a new war against the successors of the Ancient Warriors - the Wild Force Rangers. he decided to have Zen-Aku freed in hopes of using him against the Rangers. Zen-Aku awakened, and swore to get vengeance against the Rangers for the evil deeds performed by their predecessors. His memory was hazy, with only a vague recollection of being imprisoned by the Ancient Warriors, none of being human and having been cursed by a mask.
Zen-Aku's overwhelming power and fierce Dark Wild Zords defeated the Rangers in each and every battle. He even managed to steal the Elephant, Giraffe, Black Bear, and Polar Bear Zords from them. With each battle, the Rangers discovered more clues about Zen-Aku's secret, and Zen-Aku regained bits and pieces of his memory, particularly when he was around Princess Shayla. To prevent him from remembering the past, Nayzor implanted a bug into Zen-Aku's head which would further cloud his memory, and strengthen his resolve in destroying the Rangers. By this time, the Rangers had already figured out Zen-Aku's true identity, and sought a way to restore it.
The spirit of the Animus Megazord appeared to Zen-Aku twice, to try and make him remember his past. Animus even created an eclipse to cut off Zen-Aku's power source, and allow the Rangers to reach through to Merrick for a brief period. Neither plan worked, and Zen-Aku returned, still seeking vengeance. Finally, Princess Shayla determined that the power of the curse lay within the Dark Wild Zords. To break the spell over Merrick, the Predazord needed to be defeated in battle. To accomplish this, the Rangers were assisted by new Wild Zords, the Rhino and the Armadillo. With these new Power Animals, they formed the Wild Force Megazord Striker mode, which was able to take down the Predazord, scattering its Animal Crystals. Amidst the chaos lay Merrick, restored back to normal, with a broken wolf mask nearby. The Rangers asked Merrick to join them, but Merrick refused. He was not ready to forgive himself for his evildoings as Zen-Aku, and did not feel worthy to join the team.
Merrick later risked his life by taking on Master Org and the Quadra Org to get back the Animal Crystals he had stolen as Zen-Aku. He was getting beaten badly, until his old Wild Zords finally answered his calls and came back to him. They gave him the Lunar Caller, giving him the ability to morph into the Lunar Wolf Ranger. With the power of the Howling Wolf behind him, Merrick defeated Quadra Org, and then Nayzor, retrieving the four Animal Crystals in the process, and returning them to the Rangers. Merrick remained a recluse, helping the Rangers when necessary, but unwilling to get close to them, or to pursue the romantic feelings he has for Princess Shayla. Instead of living on the Animarium, Merrick worked at Willy's Road House, where he did chores in exchange for room and board. He knew when the Rangers needed him when the wind would blow, and warn him of danger.
As time went by, Merrick couldn't help but grow closer to the Rangers, and eventually accepted them as friends. After Master Org was defeated, he gave up his Ranger powers and left Turtle Cove to travel the world. He was joined by Zen-Aku, who also sought redemption for his evil deeds.