Hippos. Problem solved.I agree with you, and I wracked my head as to what kind of mount would fit them as a race. Most creatures native to Outland do not really fit being a Mok'Nathal mount (ravagers, hydras, felboars) and other beasts are already other race's mounts (wolves, talbuks, elekks).
Remembering what Plainsrunning used to be back in vanilla, I think a revamped concept of it would fit the Mok'Nathal greatly. Then again, a hulky orc-like race doesn't quite fit the bill of a super speedy runner.