This is an unused look. This appearance has never been available on a player mount.
Notes:This is actually the same skin as the Warlock Felsteed mount, which is also available on the regular horse model. It can be seen on the Fel Steed in Shadowfang Keep. Because of its resemblance to the former, it's probably much less likely to become a mount.
While we don't normally document saddleless versions of mount models, the introduction of Gilneas "Mountain Steeds" means that these horses are now valid mounts.