It seems to me that the first monster that Tang Priest met was the Monkey King. Tang Priest has always said that he came from the East to the Tang Dynasty and went to the West to worship Buddha for Buddhist scriptures. Therefore, his journey to learn from the scriptures should start from leaving Beijing, and the first monster he met after leaving Beijing was the Monkey King. In fact, the first creatures that Tang Priest met after leaving Beijing were tigers and Orion. Orion also didn't say that although the tiger is the king of beasts, it is not refined.

So the first monster was the Monkey King, the Great Sage of the Monkey. As for why he didn't eat the Tang Priest, of course, it was because Tang Priest could save him from Wuzhishan. Guanyin told him that as long as the Tang Priest was escorted to the Western Heaven, he would not only forget the past, but also become a Buddha. So the Monkey King didn't dare to eat Tang Priest.

If you don't count the apprentice the Monkey King, then the first monster Tang Yan met was the black bear spirit. I think the reason why the black bear spirit is afraid to eat Tang Priest is because the black bear spirit knows the identity of Tang Priest and that it is an experience to learn from the scriptures. So he didn't dare to eat Tang Priest, and he didn't want to eat Tang Priest from beginning to end. Just to covet Tang Priest's cassock and hinder Tang Priest's westward journey.