From correspondence with a keeper at the Arizona-Sonora:

"The lizard most likely was a Spiny tailed iguana (Ctenosaura similis). While these iguanas aren’t native to Arizona (they are native to Sonora, Mexico), the people that worked here in the mid-1970’s thought it would be really cool to see big lizards at the big horn sheep exhibit. So, they released about 7 spiny tailed iguanas. (They didn’t know at the time introducing new animals to an area is a bad idea). Since that time the population has swelled. We have conducted a study of lizards on grounds and off-grounds and have determined that they haven’t left the grounds."