In the world of "Journey to the West," actually, there isn't a very clear boundary between immortals and demon creatures, after all, many of the deities in the Heavenly Court were originally demon creatures.
Simply put, ordinary demon creatures are just that, demons; but if one receives the patronage of a certain immortal, becoming a disciple or a mount, then those who can be enfeoffed are considered immortals.
As for the essence? Whether you're a demon or an immortal depends on whether you've been enfeoffed and recorded on the Immortal Register of the Heavenly Court.
The Yellow-Robed Monster's true identity was the Star Monarch Kuimu Wolf of the twenty-eight star abodes above, a proper deity. Presently disguised as the Yellow-Robed Monster hiding in Wanzi Mountain, he claimed to be a demon, naturally to avoid drawing attention.
Otherwise, would he be able to continue living in seclusion if he announced to the world that he was actually a deity?
