From The Stargate Omnipedia
Animal native to the planet Vagonbrei similar to an iguana, two and a half to three feet in length. The creature is a cave-dweller, having survived for centuries in the same tunnels where Morgan Le Fay likely hid herself.
The lizard is the only known surviving animal on the planet, possessing a natural immunity to a local parasite. It is also possible Morgan deliberately vaccinated the creature against the parasite, to aid anyone intelligent enough to catch and dissect it.
Morpheus - Colonel Mitchell and Teal'c hunt and capture a native Vagonbrei lizard, the only surviving animal, which carries a natural immunity to a parasite.