Demon Immortal — this once referred to members of the demon clan.
After all, Di Jun Taiyi had once listened to the Tao at Zixiao Palace and practiced the orthodox mystic sect techniques as disciples of the Taoist Ancestors.
Thus, the demon clan they created, any clansmen who cultivated the Immortal Dao techniques to become immortals, could be called Demon Immortals.
To put it simply, Demon Immortal is actually Human Immortal. After all, the word human in Prehistoric World does not represent the Human Race, but all living beings between Heaven and Earth.
However, later the Human Race became too powerful, successively producing the Three Sovereigns and Five Emperors, as well as the nine supreme beings like Da Yu, which led to the Human Race redefining the Human Path. Hereafter, unless specially stated otherwise, human referred to the Human Race, not all the beings of the Human Path.