Wyvern

The Wyvern are the distant cousins of Dragons.

History
As the Wyvern were related to Dragons, Dragonlords were able to command them. (The Eye of the Phoenix)

Appearance
Wyvern are similar in appearance to Dragons. However unlike the Dragons, who are very large, the Wyvern are only slightly larger than a Human, Wyvern are also more birdlike and have red eyes. As they didn't breathe fire when fighting Merlin, Arthur and Gwaine, it is likely they don't have that ability. Unlike Dragons, the Wyvern were never shown talking, suggesting they are more animal-like.