I honesly think mermaids exist. They are legendary aquatic beings with a human upper body and a fish tail lower body.
Hey, I'm not one to judge. They just want to be part of our world, right? But mermaids are the good creatures, right? It's sirens we'd have to worry about.
I don't know what a siren is. Mermaids were first mentioned in ancient Assyria. 
Are they part of the mythology of a lot of ancient cultures?
Mermaid
In folklore, a mermaid is an aquatic creature with the head and upper body of a female human and the tail of a fish. Mermaids appear in the folklore of many cultures worldwide, including the Near East, Europe, Asia, and Africa. The first stories appeared in ancient Assyria, in which the goddess Atargatis transformed herself into a mermaid out of shame for accidentally killing her human lover. Mermaids are sometimes associated with perilous events such as floods, storms, shipwrecks, and drownings. In other folk traditions
Susulu (mythology)
Susulu (Susuna or Susona) is a legendary aquatic creature with the upper body of a female human and the tail of a fish. Also known as Suna or Sona. She is the daughter of the Sea King.
Similarities with Mermaid.
They are conventionally depicted as beautiful with long flowing hair. Susulus appear in Turkic folklore as unlucky omens, both foretelling disaster and provoking it like mermaids. Susulus have also been described as able to swim up rivers to freshwater lakes. They are usually