Update 44 brings the Keep on the Borderlands adventure pack, which adds eight new dungeons, a new adventure area, a new public space, and new named loot. As with other packs, this is either free to VIP Members and Season Pass holders or available for individual purchase in the in-game store.
And yes, there are now mounts in DDO. These new pairs of horse pants can be earned either by exchanging Quaal Feather tokens found in the Keep in the Borderlands pack, by getting a roll of at least 96 on Daily Gold Rolls, or as a free item in the Keep on the Borderlands Bonus Items Bundle — specifically a Fast Mount. On the subject of mount speed, Normal speed Mounts can be upgraded to Fast Mounts with the use of an Enchanted Horseshoe, which is a rare drop found in the treasure chests of Epic-level dungeons in the new adventure pack.
There are other updates to things like quests, items, and adventure areas whether you buy the new adventure pack or not, so make sure to look over the latest release notes for more details.