I think they will be popular enough even without a specific mount but well I tried to think what this could be but it's pretty hard..

Dragons will be all overs the place so the DRK getting a dragon too would be too much.
PLD and WAR have "normal" animals as mounts and they are connected to their PVP gear look.
The connections PLD-White Lion (Hero, Leader, Light, Honor etc) are intuitive but the WAR-Bear are not (..rawr? being reddish?), at least to me, and the only things I see true for both is they have big powerful looking animals. What kind of animal could be connected with a DRK that for sure is not like the traditional FF DRK? Someone proposed the hound and it's not a bad idea as it can be black, ferocious and connected with darkness but what about his probably blood related skills? Bats are NIN's, insects would be eew.. other than that the DRK is sometime proposed as an opposition of the PLD so a big black feline would seems appealing.