Xevion (also spelled Zevion) is a non-playable boss monster who is the main villain of Monster Rancher 4. It is assumed because of the name and the general shape that it is an evil, twisted type of Xenon, possibly with a Zan or Thine sub-type (because he has four sharp appendages on his back). His original form is much more insect-like, with large shoulders that have two eyes in each (he also has four eyes on its face, giving him a total of 8), a long, spiny upper body and lower body that looks like a giant caterpillar. This part of his body is covered in tubes and wires that have been used to pump him full of artificial power, and hold together the conglomeration of monsters and mechanics that have created him.
When you first come upon him, he is still wearing his armor from his battle with Suzaku, called his Battle Mode. His armour has giant golden insignias on the shoulders that look like the Eye of the Pharaoh, and fan-like plates that deflect energy back at the opponent.
It is assumed that Xevion was one of the first artificial monsters created during the Pendant experiments, and once it went awry they tried to dispose of him. What they did not expect was that Xevion would use his techno-organic powers to draw other modified monsters to himself. Somehow he was able to rewrite the circuitry of these monsters to make them turn on their human captors, and he formed an army called the Dark Ones (much like the Baddies who came later).
The Dark Ones tried to overthrow the humans, and in the process started the first in a long line of wars called The Dark Times. Xevion was eventually defeated by the efforts of a young Tochikan princess, who rallied humans and monsters alike, including the Phoenix lord Suzaku. However, the wars were far from over, as humans had now realized that they could utilize monsters not just as pets but as weapons of war. They re-engineered Xevion's control technology to create instruments called Anima Recorders, that allowed them to bend the will of monsters.
After several more wars, the Pendant scientists finally had enough data to create the ultimate weapon: Moo. They thought that because they had learned from their mistakes with Xevion that it would be different this time, but it wasn't.
In the Games
Thousands of years later monster modification has been outlawed for quite a while, and the only surviving manuscripts about such things have been burned--until a student at the Elives School discovers the Tome of the Black Arts, which describes in detail the work of one of the original scientists, Nahze. This brings the issue back into play again, as the boy is manipulated into modifying more monsters to be used by Xevion when he rises again. Over the course of several years the boy works in secret, and eventually goes on a search for the body of Xevion himself.
It turns out that Xevion was never actually destroyed. During his battle with Suzaku, his body somehow became lodged in the depths of the Kawrea Volcano (This coud possibly be why Phoenixes are sometimes sighted there, perhaps guarding the Dark Lord to make sure he does not escape?). Once enough monsters are modified, Xevion uses his technological influence to make them attack the humans, and plans to have Wit and TeeTee free him. Fortunately this is prevented once more by Phayne, Rio and the guidance of Suzaku, who knock Xevion down a chasm into the lava, when he is buried in rubble.
There are several similarities between Xevion and Master Moo, and it could be that the story written for Xevion in Monster Rancher 4 was a way to acknowledge the events of the anime series in the video games. Xevion was an experiment gone wrong, has a way to control other monsters, and even has two different "modes" that are a bit like the "forms" of Moo. When Xevion is in battle mode, his armor is even the distinct purple, white and black--the colors of Moo.