Mermaids are legendary aquatic creatures, so their existence has never been proven, or disproven for that matter. The US National Ocean Service stated back in 2012 that there is no evidence that has ever been found to prove that they do exist. The first stories on mermaids appeared in ancient Assyria.
It is believed that the first "mermaids" seen were actually Manatee's. AKA sea cows. I dont know how they thought they were human like but... that is how the story goes. An actual half human/half animal creature does not, nor ever has existed outside of story books.
No. A mermaid is legendary with the upper body of a human and the tail of a fish. Mermaids appear in the folklore of many cultures worldwide, including the Near East, Europe, and Asia. The first stories appeared in ancient Assyria, in which the goddess Atargatis transforms herself into a mermaid out of shame for accidentally killing her human lover. Mermaids are sometimes depicted as perilous creatures associated with floods, storms, shipwrecks, and drowning. In other folk traditions (or sometimes within the same tradition) they can be benevolent, bestowing boons or falling in love with humans.
Mermaids are pure myth. Its impossible for something to be part human and part animal. Mermaid myths come out of stories from lonely, sailors suffering from poor nutrition seeing strange sights they couldnt explain. Like manatees and dolphins.