All mounts will work as ground mounts in Warlords of Draenor!

From: http://www.mmo-champion.com/content/3836-WoD-All-Mounts-Work-as-Ground-Mounts-Launcher-Feedback-Blue-Tweets-TCG-Art
This means that Gladiator mounts earned in Arena will finally be usable inside Arena (and BGs and non-fly zones). Awesome news imo.