Mar'kak were non-sentient creatures native to the planetKar'tek (Earth designation TC9-173) which were the only other living creatures on the planet after the Volca.
When the Goa'uldMars was exiled to the planet, he discovered that the Volca could not be used as hosts and so instead he created cyborg bodies which housed Mar'kak as their biological components. He transferred himself to one of these Mar'kak, and had his Jaffa do likewise with their symbiotes so that they could continue to serve him. Although many Mar'kak were used as hosts from that point on, some still remained Goa'uld-free and were instead used as pets. (SG1: "Fall of Rome 1")