With that leaf pattern and those thorns, I'd say some form of Acacia. There are a number of species that are native to TX.

You're right, Gardengal, it does look like an acacia of some kind. The leaves have a definite bluish cast to them. Any ideas on what variety it might be? There aren't any acacias close by that I know of, but the seed could have arrived in city mulch.

I'm not adverse to acacias, even thorny ones. As Omar once remarked, after the tree is tall they aren't a problem. Just don't be climbing it ... lol.

whats growing in the background?

St. Augustine grass ...

It was suggested on the "Name that Plant" forum that it is Acacia farnesiana, or 'Huisache', which is a common tree around San Antonio.