It us currently no longer obtainable!
To get it, you simply need to complete the pre-patch quest chain. This is a three-week event that opens up a new part of the chain every week, but you can catch up later if you miss a week. The quest chain starts from a pop-up you get when you log on to a level 110 character during the event, and the first step is to talk to Sylvanas in Orgrimmar.
Just keep following the quest chain, and at the end you will be awarded the War-Torn Reins of the Undercity Plaguebat. Use this item to learn the Undercity Plaguebat mount, which is then usable on all your Horde characters.
If you've done the quest chain on an Alliance character and gotten the Teldrassil Hippogryph mount, then this mount will also be available to your Horde character without you having to do the quest chain once more.