Mentioned in "Brief Candle"
Pelops is the Goa'uld responsible for the creation of the Jaffa. Like several other Goa'uld, he became fascinated with the newly-discovered Humans, and set about to improve them even further. Eventually, he created the Jaffa, whose purpose was to increase the likelihood that a host blending would be successful. The other System Lords eagerly sought this technology, and Pelops reluctantly shared his findings with the greater community, in order to avoid being destroyed if he tried to keep it for himself.
Unlike his greatest rival, Nirrti, Pelops did not seek to create better hosts for Goa'uld. His reasoning was that Goa'uld do not conquer planets; Jaffa do. So Pelops became something of an isolationist, retreating to his small holdings on the fringes of Goa'uld space and continuing his experiments. The other System Lords saw him as being among the least politically devote among them, and most did not consider him to be a threat. This is not entirely true; during his solitude, he has developed a powerful breed of Jaffa known as Spartans, and Pelops has slowly been attacking lesser Goa'uld and stealing their resources. He is close to gaining enough power to challenge his peers openly.
It would not be accurate to call Pelops "good," or even "less evil," but his human subjects generally have better lives than many others under Goa'uld rule. This is because Pelops has no interest in dominating slaves; rather, he wishes to someday dominate his equals, the other System Lords.
In an effort to study Human physiology and evolution, research presumably geared toward improving Goa'uld hosts, Pelops implanted the people who settled on Argos with a nanovirus that aged everyone one year in the space of a single night. The virus was transmitted through physical contact, and when Colonel Jack O'Neill unknowingly married Kynthia, an Argosian woman, he was infected on his wedding night and consequently aged at a rapid rate. The source of the aging was identified and the controller for the nanovirus discovered in the base of Pelops’ statue. Once the device was deactivated, the Argosians began to age naturally and O'Neill was returned to his correct physiological age.
Pelops had not made an appearance at Argos for a very long time, and while the Argosians were initially horrified at destroying the image of their god, once they understood what was being done to them by the nanoviruses Pelops gave them, the Argosians embraced Pelops' downfall and rejoiced in their freedom from his tyranny. His current status is unknown. (SG1: "Brief Candle")