Brahmin

Brahmin are mutated brahmin cattle with two heads. 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. A bunch of brahmin made their way to the states long ago for crossbreeding purposes. When the bombs fell, brahmin mutated and grew two heads. They also have 8 stomachs, twice as many as normal cows.

Biology
The brahmin are quite hardy. They are also a delicious toasty brown, as you'll notice in the picture to the right. 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 (Yes, yes, the brahmin in the Fallout 2 opening movie doesn't have this problem, but it's sick and dying, so it doesn't count.) The heads distinguish the genders, and some brahmin have either both male heads or both female heads.

Diseased Brahmin
Some have reported seeing such a creature called a "Mad Brahmin", which is believed to be a Brahim with a disease similar to the pre-war disease known as "Mad Cow Disease". Little is known about this disease, and none know whether or not it is contagous. All that is known is that the disease causes the Brahmin to become hostile to all who it sees. Encounters of Mad Brahmin have commonly been reported near the entrance to Vault 101.

Function
Brahmin serve a vital environmental niche in Fallout - they form the foundation of survival for many species in Fallout, most notably, humans. They also form the backbone of the NCR economy. They also form the backbone of the New Reno "NCR brahmin rustling" economy. Brahmin can pull carts, old cars, plows, and dead bodies. Brahmin can be driven into herds then used as stampedes on rival tribal villages which is better than a Delayed Blast Fireball any day of the week. Brahmin can be worshipped. Brahmin hair can be woven into bags and ropes. Brahmin hide can be stretched over wooden or metal rods to make canoes, if you're into that sort of thing. Brahmin sinew can be used for bowstrings or thread for stitching. Brahmin droppings is great fertilizer (and fuel for campfires). Brahmin meat is delicious - well, only because no one in Fallout knows what a succulent Pre-War steak used to taste like. Brahmin are a source of milk that is like modern day milk, yet terrifyingly different. Brahmin can be tipped over in Fallout 3 by going into sneak and pressing A on them. Brahmin bones can be used as clubs, knives, arrowheads, eating utensils, hoes, or even dice, and their skulls look really scary dotted all over the desert landscape. Their horns can be turned into drinking horns if you feel like getting medieval. Brahmin can be used to distract a hungry deathclaw. Brahmin fat makes decent soap. Their hide can be used to make tents (or tipis/teepees), clothing, belts, saddle bags, shoes, leather armor, or a bizarre brahmin-looking disguise so you can sneak up on other brahmin and listen to their conversations. You can also use their tails as fly brushes or paint brushes, depending on your level of artistic talent.

There are domesticated brahmin and wild brahmin. Wild brahmin can forage for themselves, and they can be found across the wastes, gathered into small herds, wandering here and there, munching on the dirty weeds scattered throughout the desert. Fortunately for the ecology of the wasteland (and the survival of their species), brahmin can go for long periods of time without water - they don't need much to survive. They have a strong sense of smell, and they don't hesitate to stomp over any wasteland predator that threatens them or their calves - well, except deathclaws, because no one messes with deathclaws.

There are rumors that the smell of brahmin droppings is highly addictive. See Jet

Appearances in games
The brahmin appear in all Fallout games. One also appears as an easter egg in Arcanum: Of Steamworks and Magick Obscura, shown in a freakshow and described as coming from "a far away wasteland".