Here's why:
* Predicate: The predicate is the part of a sentence that tells us something about the subject. In this sentence, the predicate is "look rather slimy and repulsive."
* Predicate Adjective: A predicate adjective modifies the subject of the sentence and follows a linking verb (in this case, "look").
The sentence is saying that "these creatures" are "slimy" and "repulsive" in the eyes of someone.