a unicorn is a start-up that has grown out of the funding stage and is now very profitable, meaning its initial venture investors made absolute bank. it is by definition extremely rare, but that doesn't stop every startup from saying they're a unicorn to attract investors.
it's not a label you can realistically apply to a company until after it's too late for it to be anytrhing other than a descriptor of the past, otherwise you'd have a very sound investment strategy on your hands.