Technology start-ups worth over $1 billion (£758m) were once so rare that they were named after a mythical creature only found in fairy stories. But these “unicorns” have become much more common in recent years, and today there are 315 across the globe, a figure which has more than doubled in just four years. This explosion of high-value companies shows no sign of abating either – all of the following companies are tipped to top the billion-dollar mark in the near future. We take a look at who they are, what they do and how much funding they've had.