What is an atonym for fish?

The term "atonym" is not a recognized linguistic term. It seems you might be looking for a word that means the opposite of "fish," or perhaps a word that is a close synonym but has a slightly different connotation.

To give you a better answer, could you please clarify what you mean by "atonym"?

For example, are you looking for:

* An antonym: A word with the opposite meaning, like "bird" or "mammal"?

* A synonym with a slightly different meaning: For example, "seafood" or "aquatic creature"?

* A word that describes something that is not a fish: For example, "land animal" or "plant"?

Once I understand what you're looking for, I can provide a more helpful answer.