Brahmin

Real-World Brahmins
In the real world, Brahmin are a kind of cow native to India. So important is this cow to the people of India, that the priestly caste shares its name. More generally, any group of powerful people in a community are often referred to as "Brahmins."

The Fictitious Fallout-Brahmins
In the world of Fallout, two-headed brahmins are the only strain of cattle capable of surviving the hostile environment of post-war Earth. In Fallout 2, your character even has the chance to help out a wounded brahmin in the town of Modoc. Also in Fallout 2, your character will hear incredulous and unconvincing rumors of one-headed brahmin. Remember, do not tip the brahmins. Because if'n you do, out you'll go!

Brahmin appear in-game to have one male head (with horns) and one female head (without), but apparently brahmin also exist with both heads of the same gender.