The revenant dragon is a level 126 revenant. They are, however, much stronger than a player of similar level. It is the only members-only revenant. Revenant dragons can hit extremely high and have moderate defence against player's attacks, so it will not go down as quickly as a revenant of a similar level.
They have the ability to respond rapidly to a player's protection prayer. They can also inflict heavy damage (comparable to a similarly levelled player killer) from any corner of the combat triangle.
Unlike other revenants, revenant dragons are poisonous.
Revenant dragons are less likely to be killed due to the fact other revenants are much weaker, the revenant dragon is poisonous, and the revenant dragon spawn is a PKer hotspot. However, revenant dragons are more likely to drop Ancient Warriors' equipment more often than its other revenant counterparts.
An anti-dragon shield or any other equivalent is not needed when fighting these, as they do not exhibit a dragonfire attack.
WARNING:The Forinthry Dungeon is a PVP-enabled area, meaning it is entirely possible for other players to attack you while fighting the revenants in the dungeon. Do not bring equipment that you are not willing to lose.
Revenant dragons had two heads before the dragon graphics update on 5 July 2010.
It is very possible that they are the spirits of white dragons. On examination of the Third Age ranger armour, it says it is made of white dragonhide. These dragons would've then existed around the time of the God Wars, and have gone extinct/into hiding shortly after.
Revenant dragons used to have low defence for a while after the Evolution of Combat. After a hidden update, their defence level has increased dramatically, even more than a revenant knight.
Revenant dragons are not shown in Jagex's bestiary. This is most likely an oversight.