Hello, I will try to help answer your questions. Joan has given you good information about snakes that are native to where you live, and what can happen to dogs that are bit by copperheads and rattlers. The most important thing here is your dog and how she is doing. If she continues to be lethargic or there is any swelling, I strongly suggest you have her seen by a veterinarian. If this was a venomous bite, she will most certainly need antibiotics and pain medications, and possibly fluid therapy as well. Many dogs can recover from these bites with help.
As far as the kind of snake, it may not be possible for any of us to positively identify it for you without a photograph of the actual snake. The changes that have happened in your dog (swelling, lethargy) are certainly common with venomous snake bites. The copperhead and rattlesnake are species common to Texas and venomous. There are lots of snakes that sound like what you described that are not venomous (coachwhip, Texas brown snake, etc). And finally, there are people that collect snakes as pets, sometimes venomous snakes from other parts of the world, and either have them escape or release them. In most parts of the country these animals perish once winter weather sets in, but if you have an appropriate climate, like Florida, Texas, California, then they may persist in the wild. Without seeing the snake or at least a picture, I would be quite challenged with identifying the snake that bit your dog. I hope this helps - good luck with your girl.