When a unicorn is born with a mutant deficiency that adds an extra horn to its head, thus becoming a two horned unicorn, or twonicorn.
Person 1: I saw a unicorn today!
Person 2: Even better, I saw a twonicorn!
Person 1: What's a twonicorn?
Person 2: A unicorn with two horns.
Person 1: lol
Twonicorns are mystical creatures similar to unicorns, but with two horns (hence the TWOnicorn bit). They are, however, entirely their own species. Twonicorns are quite shy by nature and are so few that many people do not believe in their existence. This may also be due to the fact that that they reside in the countryside in carefully kept holes in the ground, out of the sight of prying human eyes. These holes are where they hoard any sparkly items they find and host the most magnificent tea parties.
Person 1 (singing): BUT AS SURE AS YOU'RE BORN, DON'T YOU FORGET THE UNICORN.
Person 2: What about twonicorns?
Person 1: Twonicorns? Twonicorns don't exist!
Person 2: Yes they do, I had a tea party with one last week.