This is an unused look. This appearance has never been available on a player mount.
Notes:This is a black version of the Illidari Felstalker, which is the Collector's Edition mount for Legion.
While this colour hasn't been assigned for use by the mount, there's no reason for it not to be. The model just uses the standard red felstalker skin, which means it could just as easily use the black or green ones.