This mob does all the things a rare spawn does. It spawns, rarely (1 to 2 hour respawn) and has a very low drop chance on a mount.
However, it is currently marked as a regular elite which means if you aren’t one of the firs 5 to tag it (and the group size of people camping this thing can get quite large) you’re not eligible for loot.
I think not giving this mob a silver portrait, and allowing everyone to tag it was probably an oversight, as if it was deliberate, it seems unnecessarily antagonistic to mount collectors.