Charlie isn't called a "unicorn" because of a horn on his head; it's because he's so rare. He's a male calico cat.
He was found in a Humane Society facility in Loveland, Colorado, which posted a Facebook post about him.
Almost all calico cats, which are tricolor, are female. This is because the genetic determination of their coloring is on their X chromosomes, according to the Newsweek article about Charlie:
"Female mammals have two X chromosomes (XX), while males have one X and one Y (XY), inheriting the Y from their fathers. In female calicos, one of the colors is usually due to their mother's X chromosome, and the other color is due to the father's X chromosome. Male XY calicos therefore cannot develop the patchy tricolor coats, as they only have the gene for one of the colors."
According to that article, Charlie's unusual coloring could also be the result of Klinefelter Syndrome, in which males have an extra X chromosome (XXY), accounting for the extra color. It may also have arisen from chimerism, in which different cells in one individual have different genetic codes, or from a random mutation of skin cells.
If you're interested in adopting Charlie the Unicorn, you're out of luck. There have already been "overwhelming requests" for adoption, and applications are no longer being accepted. Here's hoping he finds a great home and lives a healthy unicorn life.