Well, I have an extremely talented and creative friend, who also happens to be an expert on all things turtle.
Here's what she told me:
That guy you saw and photographed was NOT native. He was probably someone’s pet, who decided to get rid of him when they realized how big he was getting and how much energy he had. It is an African Spur-Thighed (official name : Sulcata). He was probably only 3-5 years old. They grow to be the 3rd largest species of tortoise in the world – hundreds of pounds. They are suited to living in really barren conditions where food is scarce. You can tell this was someone’s pet because of the “spiking” of the scutes on his shell. That means he has been getting WAY more food than he needs- primarily too much protein. He might survive in south Texas climate, if he can avoid being hit by a car. They don’t hibernate, and therefore can’t handle cold temps at all (note from Tracye: No problem there! Texas doesn't even know Fall is here yet!). They are extremely popular in pet stores because they are cheap, very outgoing, and as babies – are small and cute. It is sad that so many are sold without the buyer really understanding what they are getting into. As a result, many end up being released and then die in the wild. Like I said, that guy will get big- think galapogos big, he will never hibernate, he will get strong, and is a digger. If you try to keep him indoors, he will knock holes in the walls, wanting to get out and run. As you saw, they are naturally very active and migratory- covering miles. If you try to keep them in your back yard, they will burrow large dens- 3 x 5 foot underground rooms that cave into your yard. I think they are cute as heck, but really don’t make good pets. Many zoos across the country have several Spur-thighed. People find them in the wild after irresponsible buyers can’t deal with them any more.
So. There you have it. I found it fascinating information, and sad. I really would hate to see him dead on the road. I really wish I had been able to (somehow!) trap him and take him to the zoo. But this was when Ike was headed for us, and the town was nearly empty!