One of the complaints heard from the Horde is that they have fewer buyable mounts than the Alliance side, making it intrinsically harder for them to gain the 100 mount achievement. 3.1 will see some steps taken to amend that gap, as the Horde will now be able to purchase a Black Wolf, a White Kodo, and a Black Skeletal Horse.

Some Horde are still not satisfied, saying that at least one of those mounts should be Epic to truly equalize numbers, but for now, there's at least more easy mounts to get toward the Red Dragonhawk (Which is honestly a lot cooler looking than the silly blue one Alliance get, seriously).
Rumors have also been persisting over the past few days that the Horde will finally be getting an Un'goro Ravasaur as a counterpart to the Winterspring Frostsaber for Alliance. This has been compounded by the discovery of a Venomhide Ravasaur mount and a Ravasaur related quest in the 3.1 PTR files. However, Mor'vek himself seems to be unaccessible, and Zarhym has recently gone on record as saying there will not be a Ravasaur Mount in 3.1. It seems, though, that they're at least working toward the idea. Maybe we'll see one in 3.2.

Here's one more bonus to accompany that whole mounts can swim thing in 3.1: An actual swimming mount. In 3.1, Fishers will be able to fish up a new mount from Northrend fishing schools. As to the identity of the mount, a Coralsheel Turtle mount has been spotted in the 3.1 PTR files, and Zarhym has confirmed that it's the real deal. Between this and the aforementioned fishing dailies, it is a very nice time to be a fisher in 3.1.

If nothing else, between this and the tournament, it looks like we'll be seeing a few more Dragonhawks flying around. Good luck, mount collectors!