White dragon

The White dragon is a monster speculated to have inhabited Gielinor in or around the time of the Third Age. Evidence is provided that they once existed by the examine option of the Third age ranger armour, which says that it is made out of white dragonhide. Currently, this is the only race of dragons known that have officially become extinct. Like many other species and races that were influenced by the God Wars, the white dragons were likely forced into hiding or became extinct. It is suggested that the Revenant Dragons inhabiting the Forinthry Dungeon might be the ghosts of white dragons due to the fact that they used to have two heads, which is unlike any other type of dragon known, but this is no longer true. Another reason is that many species became extinct during the God Wars. However, any connection between revenant and white dragons is only speculation.

 A question asked about the White dragon(s) 

Mod Fetzki: But white dragons don’t exist... ;)

Mod Mark: Potentially. We have all sorts of future plans to do with dragon variants.

Even though Mod Mark has said they have future plans with dragons they cannot create White Dragonhide as this would render Third-age ranger armour useless and inexpensive, unless they would make it so that third-age ranger armour was too experienced for even master crafters (99 crafting).
 * It is possible the Frost Dragons are White Dragons, due to the colour of the Frost Dragon mask is white.