This is about the anime character. For the monster itself, see Gray Wolf.
Gray Wolf is Tiger's younger brother in the Monster Rancher anime series. He was captured by Moo and turned into one of his Big Bad Four, making him the third of the chief villains that the Searchers defeat. He controls the center of the Northern Continent from his ice castle. His fur is white and dirty white in contrast to his game counterpart where his fur is in a shades of gray. He initially has a Moo's insignia at the center of his chest until Tiger stabs with its horns to break into pieces to leave him dead and later it was not seen after being reformed.
His debut appearance was in Episode 7 during a meeting between Moo and the Big Bad Four. It becomes clear that Tiger assumes his brother to be dead, but after a fight with the Black Worm troops Tiger learns from one that Gray Wolf sent them after him.
In Episode 27, the Searchers reach the top of a mountain where he challenges Tiger to a duel. They seem evenly matched in techniques, so they decide to use only claws. When Tiger attacks, Gray Wolf whips him with his tail, blinding him. Tiger reminisces about how he taught his brother that trick long ago, and uses his horns to stab Gray Wolf's baddie crest. With the crest broken, Gray Wolf's right mind returns long enough for him to say goodbye to his brother. He then becomes a Lost Disc.
At the end of Season 2 Gray Wolf was shown turning back into a Mystery Disc, and in Season 3 he was resurrected by a girl named Nina. Together, they and Nina's Hopper set out to win the Legend Cup competition, and end up facing the Searchers in the Greatest 4 battle. Gray Wolf squares off against Tiger, and even though he knows his brother pretends to be tough, he might throw the match for him. So on the field, he challenges Tiger to leave their past behind them and fight fairly. Tiger wins the competition, and the two part on good terms.