The Rhinoceros King had willingly lowered his cultivation level to match Wukong in a challenge of supremacy.
Though the two could be considered peers, they hailed from different sects. The Three Pure Ones had always had their disputes, and once their disciples started fighting, it was a real battle. Wukong was a disciple of the Spiritual Treasure Celestial Venerable, and the Rhinoceros King was the mount of the Morality God. When these two fought, they reached a point where life and death were at stake, naturally holding back somewhat, but this battle was also about defending their sects' honors!
Since his birth, the Rhinoceros King had followed Laozi, braving countless storms with him, and when they traveled west through the Hangu Pass, he had even been infused with great fortune!
