He is a gigantic dragon-like monster that scientists created for war, but he was far too evil and powerful to be stopped. So the scientists created the Phoenix, the only monster whose power rivaled Moo's. Unfortunately, they ended up trapped in a seemingly everlasting battle, which forces the scientists to make a device to turn all monsters into Mystery Disks.
Eventually, Holly's father found the Mystery Disk containing Moo, and Moo was merged together with him. Moo assumed a humanoid form, resembling General Durahan, except with gold armor with brown highlights, a red and black cape, long white hair, a pure black face with red eyes, a helmet with a single brown spike, and two bull-like orange horns. This 'face' is later revealed to be a sort of mask.
Moo gathered several followers, each bearing a crest called the Baddie Crest. The Baddies were the evil monsters that served Moo. The strongest of Moo's servants were the Big Bad Four: Pixie, Gali, Gray Wolf, and Naga. Moo searched fort the Magic Stone, which was in the possession of Holly, and wanted to use it to find and regain his original body.
The Searchers (the protagonists, consisting of Genki, Holly, Mocchi, Suezo, Golem, Tiger, and Hare) fought Pixie, and she eventually switched sides. Afterwards, Moo found and kidnapped Holly to take the Magic Stone. Although the others rescued her with the help of Vulcan and the Pirate Dragons, Moo found the way to his original body and arrived at the area.
While the Baddies dug through the ice for Moo's original body, the Searchers fought Gali and Gray Wolf. Gali was killed by Mocchi using his Mocchi Cannon attack, and Gray Wolf was killed in 1 on 1 combat with his brother Tiger, who sent his horn through his Baddie Crest. Right after Gray Wolf's defeat, Moo found his original body. His Baddies dug the dragon-like body free of the ice, while Naga attempted to kill the Searchers.
By the time Moo merged with his original body, the Searchers had already fought Naga, resulting in Naga getting hit with a Mocchi Cannon, hanging onto the edge of his castle on the cliff, and letting go and falling to his death. Moo attempted to invade a village, but was weak against sunlight. When the sun rose, he was seriously weakened, and retreated back to his hideout.
For most of the following episodes, Moo remained severely injured inside his hideout, and sent Evil General Durahan after the Searchers. Durahan's many servants were defeated, but he wanted the Magic Stone for himself. Once he got the stone, he betrayed Moo and nearly killed him, but by this time Moo was almost back at full strength. However, his assistant Lilim betrayed him for Moo. The Searchers and Durahan and Lilim fought on Durahan's ship, and in the end, a Weed got the stone and took it to Moo. Durahan's troops, who now served Moo instead of Durahan, shot down Durahan's ship. Durahan killed Lilim before dying himself when the ship crashed.
Moo got the Magic Stone and became immune to the sun. He attacked the Searchers just as they found the frozen Phoenix, and mortally wounded Pixie, but was prevented from killing her in a bizarre turn when he saw the true evil of his soul, after absorbing too much hatred. Moo completely lost his mind (symbolized by his original form being trapped in darkness and destroyed, and his eyes turning yellow) and began to go berserk, destroying everything in sight, even his own castle and the Baddies. Pixie was healed when Big Blue fused with her to form Granity, sacrificing his existence in the process. Monol informed the Searchers that the Phoenix was really Mocchi, Suezo, Golem, Tiger, and Hare, and so Genki fused them together.
The Phoenix fought Moo and grabbed onto him. Seeming to regain a part of his mind, Moo realized that he and the Phoenix were destined for eternal combat, and that he couldn't let the Phoenix nor himself exist. At this, the Phoenix and Moo destroyed each other, vanishing in an explosion of light.
In Season 3, the role of antagonist shifts to General Durahan, who is resurrected as a head by a Weed (who in turn becomes a cyborg), and attempts to absorb Moo's soul, which was trapped inside a disk after his death. In the final episode of Season 3, Durahan and his servants, Gobi, Mum Mew, Poison, and Weed, are fused with Moo's soul to form a gigantic warrior, with Moo dominant. Moo sucks the Searchers and Holly's father inside him, during which Mocchi, Suezo, Golem, Tiger, and Hare fused into the Phoenix again. Moo still seemed able to gain the upper hand, but when Durahan and the others give the Phoenix their energy, he is able to attack Moo's soul within, destroying it. The evil is last seen rising into the air and exploding. Thus, Moo was destroyed for good.
Moo's original form is a Dragon-type monster with black skin, white and pink reddish fur, and three-digit claws. He has a pair of golden horn which curved in the same manner with bulls. Also, his body was huge, so large that it nearly equals Tailed Beasts from Naruto series (ex. Shukaku) in terms of size. He has a squat face with a mouthful of sharp fangs (which get even sharper when he's gone berserk) and a pair of bat-like wings upon his shoulders. His tail is long and white, with a red tip, resembling that of a fox's. All in all, Moo resembles a mammal more than he does a reptile.
After possessing Holly's father, Moo takes on a more humanoid appearance, but is still quite large, nearly bigger than Golem. He wears gold and brown armor, a red and black cape, and sports a ring of white hair around the base of his neck that resembles the fur of his true form. Moo's cape is tied just below his neck and the Baddie Crests rests upon the knot. A helmet with curved horns covers nearly all of his head, exposing only a smooth and rather featureless face and two piercing red eyes. After merging with his former body, this face of Moo shatters to pieces.
- Moo was voiced by Paul Dobson,who presumably voiced Holly's Father as well.
- Moo appeared as a unique breed of dragons in Monster Rancher 2 only available on the CD Beck: Mellow Gold.
- Moo is similar to Darth Vader from Star Wars, seeing as they both have a similar appearance, deep voice and, in a way, Moo is Holly's father
- Two of the crucial episodes to Moo's quest for his original body were skipped by the Fox channel. Tiger's Battle with Destiny, the episode where Gray Wolf was defeated, is also the episode where Moo finds his original body, and Battle with the Big Bad Four, the episode where Naga was defeated, is the also episode where Moo merges with his original body.
- Despite how evil and powerful he is, and how many people are frightened of him, the name is Moo may not sound very intimidating to some, as it is both phonetically and grammatically similar to the sound a cow makes in English. However, this is a play on the Japanese word for void, meaning "nothing".
- Zevion, a dark monster that appears in Monster Rancher 4, is Moo's spiritual successor, as both have an army of evil monsters (Baddies for Moo and Dark Ones for Zevion). They both share a color scheme as well.
- The main villian of Monster Farm Lagoon is Moo once again, and this time, several level bosses are simply monsters (such as Tiger, Kikimo, and something called "Moo(Larva Mode)" which looks alot like the dragon manifestation of Moo's soul) taken over by Moo's soul. However, Moo's true form in this game has 4 wings instead 2.
- Interestingly enough, Moo has the exact same color scheme as the Ripper from Monster Rancher 4. Perhaps a sub-type?