White Dragons - White dragons are much tougher that normal dragons. They have double the health (10) and do twice as much damage. In other words, Claws do 12 points, Wings do 10 points, Tails do 6 (and reroll), Jaws do a whopping 24 points!
Even the Treasure is twice as much, allowing 2 units to promote. However, there is a price for this; a white dragon replaces 2 normal dragons and takes twice as much magic to summon. So in a 36-health game, you could either bring 2 normal dragons or 1 white dragon.
Any colour dragon summoning spell can summon a white dragon, however, it takes 14 points of magic instead of 7. See the rules for more information.