You won't "major" in Herpetology. You will major in Biology/Zoology with the emphasis on Herpetology. After you get the basic Biology and Zoology courses successfully under your belt you can get into a Herpetology program to branch off into your individual area of interest. To my knowledge, there are no specific degrees in Herpetology, at least in the U.S. The degree will be in Biology. Your Masters work and Graduate work will be related to Herpetology. As Andy said, most Universities with large Biology Departments will offer a Herpetology program. I know that Auburn University and UGA have good Herpetology programs.
By the way, you can study rattlesnakes (or any other group of animals) without being a Toxicologist. You can certainly have double majors and go for higher degrees in both disciplines. That's quite an aggressive goal, but if you can do it, go for it.