Wow.com has a few 3d models data-mined from the PTR build. One is a celestial horse, that looks like it's made from a constelation, pegasus-style. This, unfortunately, doesn't look like a very scary warlock mount, but darn-it, those pallys (the warlock's goody-two shoes cousin) could take this one for a spin.
On the other hand, this would be extremely cool as just a regular mount, especially if it could have the 310% flight.
Found this video of a orc warlock flying around on this mount: