There is no such thing. “Zebra update” is fake. Matt Cutts never told anybody anything about “zebra”.
Somebody made a joke, and even put a message that it’s a joke at the end of it:
(N.B. The Zebra update is currently ficticious. All comments made here should be taken with a large pinch of salt and then disregarded entirely)
It didn’t stop people from spreading this thing everywhere.
PS: “Wolf” update is fake as well.