What are the fabled marine creatures with a body and fish tails?

You're likely thinking of mermaids, the mythical creatures with the upper body of a human female and the tail of a fish.

There are also other fabled creatures that share similar characteristics:

* Mermen: The male counterpart to mermaids, with the upper body of a man and a fish tail.

* Sirens: In Greek mythology, these creatures had the upper body of a woman and the lower body of a bird, often depicted as alluring and dangerous.

* Nixies: Water sprites in Germanic folklore, sometimes depicted as having fish tails.

* Undines: In medieval alchemy, these were elemental spirits associated with water, sometimes portrayed with fish-like tails.

It's important to remember that these creatures are all fictional and have no basis in reality.