Greater Dragons are fearsome beasts that spawn alongside "normal" Dragons, but are far stronger. They provide similar loot but in greater amounts. They are only slightly less powerful than Ancient Wyrms.
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 a tamer's 5 control slots, and thus cannot be used in conjunction with a mount or any other pet.
As you cannot use any control slots for a mount you will find them difficult to tame solo (even with 70 points worth of physical and fire Resistances wild versions can easily hit for over 70 damage with their claws or Fire Breath attack).
An easy way to tame greater dragons is by using the Honor virtue. Being a follower of honor (second tier), and targeting yourself will keep all monsters from attacking you for about 90 seconds, but 5 minutes at Knight level. This also applies to monsters you anger while attempting to tame. This gives you plenty of time to tame one or more greater dragons, provided you have 120 taming.
Note: These skill/stat/resistance values were the highest seen from approximately 50 wild Greater Dragons. For most, it's unlikely they reflect the actual maximums.