Here's why:
* Subject: The subject of a sentence is the person, place, thing, or idea that the sentence is about.
* Complete Subject: The complete subject includes the simple subject (the noun or pronoun) and any modifiers that describe it.
In this case, "mythical creatures" is the simple subject, and "such as mermaids and mermen" is a prepositional phrase that acts as an adjective modifying "creatures."