Unlike previous PvP mounts, which have either simply required honor points to purchase, or required a top title, these fall somewhere in the middle of the field, and reward participation. As Holinka confirmed via Twitter, they are won by doing 3v3 and RBGs.
@Deontto Win 100 3V3 arena games and 40 RBG in a season- Holinka (@holinka) July 3, 2013
The requirement is for both sets of wins, so you can't just win 100 3v3s, or 40 RBGs, both requirements must be completed to get the mount. The aim, as Holinka again clarifies, is that these mounts should reward the well-rounded PvPer. They are attempting to increase participation in both 3v3 and 5v5, and these mounts are part of that. Of course, it's early PTR, and never say never, but it's unlikely that 2v2 will receive a similar reward. I get the impression that this is just the first of many incentives coming for players to get into PvP.