Here's why:
* Common nouns refer to general things or people, not specific ones.
* Proper nouns are specific names for people, places, or things.
Examples:
* Common Noun: dog, city, river
* Proper Noun: Fido, New York City, Amazon River
The Minotaur is a creature from Greek mythology, not a specific individual. Many stories feature a Minotaur, so it's a general term for that type of creature.