The plot has shifted somehow. The beast that was supposed to attack the students today was supposed to be an A-rank silver-horned rhino. Rhea would have to fight the beast being the third strongest of the class and someone who feels responsible for the safety of her classmate. And after a lot of struggle and a few flashbacks, she would have defeated the rhino.
Then what happened so suddenly for this Illusion Monkey, which was an S-rank to appear all of a sudden?
'Did Valerie's presence change something?' There is always the concept of the butterfly effect, and there is a saying that the world always tries to balance out forces to keep things in check.
In this world as well, the stronger a warrior becomes, the better the beasts would adapt and evolve.