In the game Armies of Gielinor, (found in FunOrb) Flametongue appears on the King of the Hill level in the Guthix Awakens campaign in which he is killed. His existence suggests that the King Black Dragon found in the modern Lava Maze was once not the only one of his kind.
The third and Fourth dragonkin journals suggest that there is only one King Black Dragon in existence, created by the dragonkin to try and free themselves from the curse placed on them by the Stone of Jas. As there is only one found in RuneScape, in the wilderness, that is named Shakorexis, it is generally believed that Flametongue is non-canon. However, dialogue with the Taxidermist when you show her the King black dragon head implies there may indeed be more than one. The King Black Dragon does have a brother though, known as Verak Lith which is dragonkin language for King/Emperor Superior, this means that there is more than one King Black Dragon.