In what is now Caesar's Land, the greatest threat to the towns and tribes was not the mighty Deathclaw of the Core Region - rather, it was the Giant Gila Monster. These creatures were the absolute apex predators of the region, growing to as long as 20 feet (and potentially longer), extremely fast, and with a fiery radioactive breath.
In truth, however, there were more to these creatures than met the eye. They did not simply grow gigantic as a result of radiation per se: rather, radiation rendered them extremely long-lived (though not ghoulified). As they age, they never stop growing - in fact, it has been speculated that there is no upper limit on their age, and therefore no upper limit on their growth beyond the laws of physics and available nourishment. However, growth does slow down beyond a certain point.
At birth, the creature is only as large as a normal gila monster. At this stage, they tend to run in packs. As they grow larger and larger, eventually they develop their atomic breath. At this point the pack thins out somewhat, with only the oldest creatures becoming solitary. As they grow older, they develop seemingly random mutations - an extra eye here, an extra horn there. Legend has it that the oldest of these - perhaps even dating back to before the war - have developed an alien intelligence, based simply upon the accumulation of memories and the expansion of their simplistic reptile brains, creating alien psychologies with nearly infinite repositories of memory.
Several tribes worshipped the Gila Monsters due to belief in these gods of the desert, and it was a very common sight for a tribe to have its own gila monster mascot that they would attempt to bulk up to humiliate the gods of the other tribes - oftne by feeding them way-ward travelers.
When the Legion rose to power, they seem to have finally wiped out the giant gila monsters, and their worship - though rumors persist of hissing voices that may be heard deep in the desert.