A non-humanoid unit that can be ridden by a non-Heavy humanoid unit for increased mobility and apparently combat bonus or protection. While mounted, a unit expends it's mount's move rather than it's own while crossing hexes. Many mounts are also able combatants in their own right and some are flying units.
The unit and their mount still count as separate units for damage purposes. A unit can be knocked off their mount by a decent attack, or have it destroyed from under them.
As a mounted unit does not use it's own move to cross hexes, it is possible to create a relay of mounts that allows a unit to travel great distances in a single turn. The unit would mount up, ride to the next mount in the relay, then mount that unit and ride on.
Known mounts include: