What you've got there is a plecostomus. A bottom dweller with a large sucker mouth ringed with small teeth that eats algae. It's native to South America and has been a big part of the aquarium trade for 50+ years.
People buy them for their aquariums to control the algae..they can keep the glass clean as a whistle! When bought they are only a couple inches long. However, they grow very large...I've had them grow to well over 12" in about 14 months. Some pet shops will take them when they get too large for peoples tanks but only if they have other customers with large tanks or maybe a fish pond. And so alot of prople just dump them in their local lakes, streams and quarries. To the extent that in areas they are a real problem to the native fishes. It not that unusual to see them now, expecially in places that have alot of big rocks. I've seen several large ones in Lake Travis, in Austin.