This is an unused look. This appearance has never been available on a player mount.
This amazing chimaera appeared in the early Warlords of Draenor database before being removed entirely. It was used during the Warlords alpha test before being replaced by the more Azerothian-style rylaks we see today. At this stage we have no idea when, or even if, it may return but it'll be a tragedy if it doesn't.