The Wheel Bug is known around here as the Stegosaurus bug which is just how it looks. Slow moving and over 2 inches long, but hard to spot; it certainly grabs your attention when you see it.
More common in the eastern United States, Texas is the western end of its range. Surely San Antonio is the outer limits.
It's a form of assassin bug and blows poison out the "back end". I hope this one found plenty to eat since they are beneficial in the garden.