I've seen them being seized on Animal Cops: Houston, though my dad does a lot of driving down by I-10 and swears there's plenty. They make a fairly lucrative business... llama wool is good, especially alpaca, and might give them enough money to send Teyla to school. :)

My family owns land down near the Rio Grande, and while it likely couldn't support a horse or cattle, it might do just fine with some smaller livestock. Of course, that's if the people crossing the border would stop cutting our damn fences, instead of walking around.