What is the history behind mermaid and Unicorn?

Answer 1

Mermaid's were the result of the Island Neverland becoming contaminated with "Star Stuff." Mermaid's grew from fish that got too close to the chest leaking the magical contents.
--Peter and the Starcatchers, the prequel to Peter Pan

Unicorn's were part of God's original creation, but missed their chance to get on the Ark
--Shel Silverstein, in Where the Sidewalk Ends

Answer 2

The unicorn sometimes has a familiar description. One horn, a thin tail with a tuft at the the end and legs like tree trunks sound familiar. It basically describes a rhino. I am sure some of the other images come from a desire for beauty, instead of truth.

The mermaid was what jealous housewives thought their husbands weren't telling them when they described manatees. The stories grew from there.

Answer 3

It goes so far back no one really knows. Columbus's men thought manitees in Florida were Mermaids at first. They were so scared of anything and everything in the new world.
Unicorns or one horned horse type animals are artists creations (or I think) in an expression of beauty as they sought to understand and draw.
There may have infact been one horned animals much like horses which some humans saw before they vanished. I think this is a very good theory.
Mermaids are I think just fanisty living in the magic minds of men and women.

Answer 4

Unicorns did not get to the arc on time and the last one was crying on the last mountaintop visible in the storm.

Mermaids are from a ship of female orphans which sunk, then the fish took care of them. Then when they grew up the fish mated with them and made mermaids.

There is no "history" there is only tales of them both.