Conversation so far:
What was Gildas?
Gildas makes an allegorical condemnation of 5 British kings.
Why did he condemn them?
likening them to the beasts of the Book of Revelation.
What did he say about Gwynedd?
describing Maelgwn as the "dragon of the island" is appropriate.

Question: Why was that an appropriate description?
Self-contained rewrite: Why was describing Maelgwn the "dragon of the island" an appropriate description?