Isis was a Goa'uld.
The discovery of the Egyptian treasures from the ill-fated Stewart expedition included a canopic jar that held the Goa'uld symbiote Isis. The seal on the canopic jar had been broken during transit from the wreck of the expedition's ship, which killed Isis.
An autopsy was performed on the symbiote at the SGC, and further examination of the canopic jar revealed that it was designed to function as a stasis chamber, imprisoning the symbiote presumably by Seth, according the Egyptian legend. Having found Isis in the Isis jar, it was presumed her companion, Osiris, was imprisoned similarly; however, the Osiris jar listed among the artifacts was missing.
Archaeologist Dr. Sarah Gardner ended up being the closest suitable host when Osiris was somehow released from his jar. Once he was able to return to Egypt and the sealed chamber where his raiments and those of his queen had been stored, he selected the attire and accoutrements of Isis in order to fit with his new host's body. (SG1: "The Curse")