Update: The Warforged Nightmare is now on the shop site!
Warforged Nightmare - Available Now In-Game Through the Shop
It looks like the Warforged Nightmare isn't available on the website yet, but it appears to be live in game in the US. This mount costs $30, which is $5 more than the usual $25 price.
Using the item that comes in the mail will give you the Nightmarish Hitching Post, which can be used to gain a buff that makes all of your mounts look like the Warforged Nightmare for the next 20 minutes.