Best read literally:
For the last year, Google’s work force has increasingly been under attack from a herd of unicorns.
Google, the Internet search company, has undergone specific raids from unicorns
Rodrigo Ipince, 28, a software engineer who recently left Google and was pursued by unicorns
While the unicorns typically pick off small groups of engineers at a time
Also, the unicorns say they are far more fleet-footed and cutting-edge than large organizations.