The director started our tour of these beasts by noting that mounted combat can only be done outside. No Mammoths in the living room, apparently. Riding these creatures is fundamentally very similar to using speed-increasing riding mounts you'll have encountered in other games. The three creatures available at launch will be the horse (the primary mount for the game) as well as the rhino and mammoth. The two larger mounts are only going to be available to pre-order customers for some time. Godager noted that at some point they'd slightly retool the art for those creatures and release them for live players, but some period of exclusivity would be allowed to early purchasers.
Actually mounting your beast involves an animation; the delay time we see in using mounts in other games doesn't just tick through a 'spell use' bar. Instead, bringing out your mammoth involves the beast appearing, kneeling down, and your character clambering atop the creatures shaggy back. It only takes a brief moment, but that small animation cycle adds a lot to the experience of mounting up. The larger beasts felt especially powerful as the player characters saddled up - an act you'll first be doing at level 40 (of 80 levels)
Mounts are in the Beta right now, apparently, but are only accessible to GMs. They'll eventually be released to Beta players so that bugs with the mounted combat system can be tracked down. Combat on your beast uses basically the same system as on-foot combat. The unique combat system we've previous discussed is here in full force, with special abilities matched up to your mount.
The bigger animals are slower moving and harder to turn, but also have PvP siege abilities not available to the meeker horses. Mammoths can actually slam their way through walls in the PvP siege battles that will be so important in the title's endgame. Positioning your character seems to be the primary focus of mounted combat; in PvE mount abilities will allow players to 'break' monster encampments more easily. We saw a horse using a backwards kick ability that would allow a swarmed player a critical moment of breathing space. Back in player vs. player combat, being able to escape aggressors and move your forces quickly about the board will the be a huge advantage.
Ultimately, it's not clear right now how big an element of the game mounted combat will be; by differentiating horseback combat from combat on the run, they've opened up a new and unique opportunity for players. Only time and a live service will see how much this experience matters to the average post-40 player.