This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Mares of Diomedes (Greek mythology)
The Mares of Diomedes, also called the Mares of Thrace, were a herd of man-eating horses in Greek mythology. Magnificent, wild, and uncontrollable, they belonged to Diomedes of Thrace, king of Thrace, son of Ares and Cyrene who lived on the shores of the Black Sea. Bucephalus, Alexander the Great's horse, was said to be descended from these mares.
One labour of Heracles was to steal them. In one version of the story, Heracles brought Abderus, one of his many male beloveds (eromenos), and some other youths to help him. They took the mares and were chased by Diomedes and his men.
Heracles was not aware that the horses were kept tethered to a bronze manger because they were wild, man-eating and uncontrollable, and Heracles left Abderus in charge of the horses while he fought Diomedes, but Abderus was eaten.