Several people in Greek mythology were named Diomedes. The most famous Diomedes was a hero of the Trojan war who later went on to found several cities in Italy. A lesser known Diomedes was a giant who lived on the shores of the Black Sea with his four man-eating mares; these vicious horses were captured by Hercules as part of his 12 labors. Several other Diomedes can be found rattling around in various Greek tales, confusing the matter even further.
The Trojan war hero Diomedes was the king of Argos and a famous warrior. In addition to being very skilled on the battlefield, this Diomedes also enjoyed the favor of Athena, who supplied him with advice, weapons, and good fortune on the battlefield. His role in the Trojan War varies, depending on the story, but he is generally portrayed as a major player in the conflict, and in some cases he even accompanies the warriors inside the Trojan horse.
Unfortunately for Diomedes, it appears that he was away from home too long, since upon his return to Argos he found that his wife has been unfaithful to him and he had been supplanted as king. Diomedes ended up moving to Italy, where he allegedly founded several cities and was ultimately granted the gift of immortality by Athena upon his death. In some stories, Diomedes is treated as a minor god, along with other mortals who have been given immortality by the gods.
The Diomedes with the man-eating mares was the king of Thrace, and the son of Ares and a nymph. Most stories describe this Diomedes as a giant who ruled over a tribe of warriors, and he was ultimately fed to his infamous horses by Hercules. The ultimate fate of the mares is a subject for debate, as some stories about the labors of Hercules describe them as being tamed and allowed to roam free, while others state that they were sent to Zeus as a sacrifice.
Given the very different lives and natures of these two Diomedes, it is usually easy to figure out which one is under discussion from the context. However, there are other Diomedes, such as a Trojan prince and a mistress of Achilles, who is usually known as Diomede, rather than Diomedes. Incidentally, the name “Diomedes” means “of godly counsel,” and the name is not uncommon in Modern Greece as well.