The Guardians didn't know the Dark Master was a Purple Dragon

[CrystalBlazier]

All the Guardians knew was that they were fighting an enemy called the Dark Master and they didn't know he was a rare purple dragon like Spyro. They've been fighting the Dark Armies for centuries, over a thousand years after Malefor started the war even after he was imprisoned in Convexity.

The Chronicler also mentioned that not all of dragon history was recorded, only most of it, and the records within the White Isle "date back to the beginning of time, nearly".