Based on creature type:
* Human: A mermaid is half-human, half-fish, so a human is the opposite in that they are entirely human.
* Land creature: Mermaids are associated with the sea, so any creature that lives exclusively on land could be considered an antonym.
* Fish: While mermaids have fish tails, they are also partially human, so a regular fish would be a contrast.
Based on mythical creature:
* Human: Mermaids are mythical creatures, while humans are real.
* Monster: This is a broad term, but some monsters are land-based and non-aquatic, contrasting with the sea-dwelling mermaid.
Based on gender:
* Merman: A merman is the male counterpart of a mermaid.
It's important to remember that antonyms are often dependent on the specific context in which you're using the word.