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. Like many other species and races that were influenced by the God Wars, the white dragons were likely forced into hiding or became extinct.
|“||But white dragons don't exist... ;)||”|
— Mod Fetzki 
|“||Potentially. We have all sorts of future plans to do with dragon variants.||”|
— Mod Mark 
Third-age range armour is made from white dragonhide so releasing white dragons could pose problems. Although simply making new white dragonhide armour rather than having third-age craftable is a simple solution.
Dragons and dragon-like races