Google isn't very helpful. There are several conflicting opinions. I like this one though: "Mermaid tails move on a vertical axis to propel themselves in the water, just like their dolphin and whale friends. Fish tails move on a horizontal axis. Therefore, mermaids are not fish." From: https://knightkrier.com/7284/opinion/mermaids-would-be-mammals/#:~:text=Mermaid%20tails%20move%20on%20a,Therefore%2C%20mermaids%20are%20not%20fish.