Greater Dragons are fearsome beasts that spawn alongside "normal" Dragons, but are far stronger.
They are only slightly less strong than Ancient Wyrms.
They have similar loot to normal dragons.
They can be tamed using the Animal Taming skill, and are considered extremely useful in both PvP and PvE combat. However, they take up ALL of the tamer's 5 control slots, and thus cannot be used in conjunction with a mount, or with any other pet.