The results are very different by market cap though. Half of the value on the list came from 2009 alone (Stripe and Airbnb). (Obviously it takes a while to become a unicorn.) Here's the sum of the unicorn market caps by year:

  2005	1b
  2006	(none)
  2007	11
  2008	5
  2009	50
  2010	2
  2011	(none)
  2012	10
  2013	6
  2014	16
  2015	1
  2016	1
  2017	1