Just like in real life, unicorns often turn out to be illusions.
A private company that someone, somewhere decided is worth a billion dollars. A billion dollar valuation is not necessarily related to whether or not a company is profitable. See WeWork, Uber and Lyft.
“From an investor perspective, particularly for traditional venture funds that are $600 million, the way the math works is you have to be investing in unicorns. If you have a billion dollar fund, you have to own a certain amount of a $10 billion company to be able to be doing a good job.”
-- Aileen Lee, venture capitalist who coined the term unicorn in her 2013 report on billion dollar startups