Stories from Greek and Roman myths were re-told and illustrated in medieval manuscripts. When the Greek hero Jason and his soldiers, the Argonauts, arrived at the sacred grove of Ares to take the Golden Fleece, they had to defeat a giant serpent or dragon. The sorceress Medea put the dragon to sleep so Jason could take the fleece. This medieval illustration depicts Jason beheading the dragon. A Khalkotauroi, a fire-breathing ox that Jason used to plow the island of Colchis, is also depicted.
To learn more about the history of dragons, see The Evil, Medieval Ancestors of Modern Dragons
15th century English manuscript; Courtesy of the British Library Catalogue of Illuminated Manuscripts.