iskandar
In Fate/stay nigth Iskandar, or well Alexander the Great was was summoned in the 4th Grail War as a Rider and it was probably the most powerful servant in the conflict (due also to the possession of a great noble phantasm). He bravely fought against Saber and was defeated by Gilgamesh. Alexander is mostly known as man who conquered most of the known-world at his age (about 340 b.c.). In only 12 years he created an Empire which was comprehending the territory between Greece and India. But he was also able to expand the greece culture in all those areas and creating the ellenistic period thanks to all those populations and tribes he was able to include in his Empire. Due to his fame, to his militare success and to his exceptional charisma, he become an almost legendary figure thanks to the many poems and stories dedicated to him and to his life. He's the perfect Rider after all and having him involved in this story as a servant is almost "natural".
Perseus
In Fate/stay night Perseus is the heroe that killed Medusa back in time, as his legend says. Rider really hates him and frequently compared him to Shinji, although he wasn’t a loser as he is. Perseus actually was a mischevious but wise heroe of the greek mithology. He was the son of Zeus and Danae and because of a prophecy that stated he would end the kingdom of his grandfather, he was oblidged to escape from his own country and found asylum in Serifo, where Polidette, the king of the isle, wanted to marry his mother. Since he wasn’t able to make any offer to the king for the wedding, he decided to take the quest of Medusa. He was luckilly helped by the Gods, who gave him the chance to acuire some powerful items (the winged sandals, the helmet of invisibility, the mirrored shield, the slaying Harpa, and and a bag to put Medusa's head into). As he was coming back from his quest he met Andromeda and he succedded as well in saving her. After many more obstacles he finally claimed back his country and become the king.
Stheno and Euryale
Stheno (forceful) and Euryale (far-roaming) are the Gorgons or well Medusa sisters. They’re both immortal (instead of Medusa who is mortal) and they’re described as young and beautiful maiden, who were looking after lost travelers, comforting them and guiding them. They appeared as pure and caring although they were somehow selfish, arrongant and presumptous and they don’t seem to care very much about their younger sister (although it isn’t so true). According to the original legend they were beautiful maidens, with golden locks and with the gift of a stunning beauty, but due to the desecration of Athena’s temple (happened because Medusa was abused there by Poseidon) they were turned into terrible monsters with brass hands, sharp fangs and hair of living and venomous snakes.