Hyena King was amongst a band of hyenas who ambushed First Squad. Lin Chung attempted to communicate with him, but he refused to listen. He then agonized First Squad with his sonic wails, only to be defeated by Mighty Ray wearing earplugs.
A story told of the origin of the Spirit Cave tells that human thieves stole from the animal lords, who in turn laid a spell on the thieves' hideout. The thieves were thereafter turned into cowardly hyenas. Although the Spirit Cave is real, it is unknown if the story is true. The fact that hyenas live in the vicinity of the cave may reinforce its veracity, though it is also possible that the mention of hyenas in the story is inspired by these hyenas.
However, the story was said to have taken place a long time ago, meaning that if it is true, the hyenas now living there can only be the transformed theives' descendants, not the thieves themselves.
Alternatively, it is possible that the story is true, but the hyenas now living at the Spirit Cave are unrelated to the transformed thieves.