Wyvern

The wyverns are magical beast and the distant cousins of dragons.

Appearance
Wyvern are similar in appearance to dragons. However unlike the dragons, who are very large, wyverns are only slightly larger than a human. A wyvern is also more birdlike and has 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 wyverns were never shown talking, suggesting they are more animal-like. Wyvers don't seem to have an aptitude for magic, too.

History
When Arthur Pendragon set upon a quest to retrieve the golden trident of the Fisher King, he travelled to the Perilous Lands and reached the King's Dark Tower. There, he was attacked by different of wyverns, with which the tower was infested. The Prince, being his life force sucked out by a Phoenix Eye, would have been killed, if Merlin and Gwaine hadn't saved him. As the wyverns were related to dragons, a dragonlord, like Merlin, was able to command them (The Eye of the Phoenix).

Morgana, High Priestess of the Old Religion, took her dying sister Morgause to the Isle of the Blessed to sacrifice her in the ritual that could tear the Veil between the worlds. The Isle was now infested with wyverns, but these creatures didn't attack the sisters or the old man who ferried them across the lake.

Upon ariving at the Isle of the Blessed, the Knights of the Round Table were attacked by two wyverns. Merlin was able to comand them to leave, but more attacked. Elyan, Sir Leon and Percival held them off, so that the other knights could complete their quest (The Darkest Hour).

Myth
Wyverns were thought to have the characteristics of dragons and birds, and to be aggressive predators, attacking anything thought to be a good meal. Their main damage came from their talons and tail. In myths, the wyverns couldn't breathe fire.