Brahmin are a caste of religious teachers in Hinduism. Does the FO4 name for cows, Brahmin, have anything to do with this? I was thinking it's possible since Hinduism reveres cows.
The Brahmin of the Fallout universe are in fact descended from Indian Brahman cattle. This is covered in the Fallout Bible 9, one of a series of design docs from the Interplay days that were leaked when the studio was going under and Fallout seemed to be all but dead forever.
Brahmin are mutated brahmin cattle with two heads. If you just said, "hell, I thought brahmin were something from India," well, you'd be right. A bunch of brahmin made their way to the states long ago for crossbreeding purposes. When the bombs fell, brahmin grew two heads. They are quite hardy. They are also a delicious toasty brown, as you'll notice in the picture to the left. They attack by head-butting or trying to gore someone with their horns, so brahmin-tippers beware. For some reason, only the left-most brahmin head has horns, which raises some curious gender identity issues.