There are several mounts you can get from rares throughout Draenor, but the Voidtalon of the Dark Star is by far the rarest, hands down. This mount is also totally unique in that it is found unlike any we've come across in the past. Forget everything you know about the Time-Lost Proto Drake, Aeonaxx, or even that Mysterious Camel Figurine - that won't help you here.
How to find the Voidtalon of the Dark Star:
In order to obtain this beautiful mount you'll need to find and enter an Edge of Reality portal. These portals can be found in various places throughout Dreanor, typically near level 100 areas where apexis crystals drop off of mops. Only one portal will ever spawn in on a particular server at a time, and the spawn time between them can be lengthy. On top of that people are still finding new spawn locations periodically, so there could still be unknown spawn points out there.
The map below will give you a rough idea of where the current confirmed spawns are located. (Scroll further down this post for much more detailed maps and screenshots of each portal location.)