A Dragon was a winged-beast often considered mythological creatures.
When arriving in a desolate forest, the Doctor, Susan Foreman, Ian Chesterton and Barbara Wright were attacked by a dragon who was slaughtered by Sir Bron of Westhold. He told them that they were common in Elbyon. Marton Dhal kept some in The Black Tower and used them as minions to fight for him. (The Sorcerer's Apprentice)