That is the special TwitchCon surprise. Mounts are playable on the show floor 
This thread was added on September 27, 2019, with posts from Jens_Erik.
Sorry for the late reply!
- Yes, there’s mounted combat
- That’s the idea
- It’s something we’re looking into, but I can’t say definitively that it will make it in
- Yes
- Horses only
Thralls won’t be able to mount up and ride, unfortunately. Mounts will only be available for players.