Brahmin

Brahmin are mutated "Brahman" cattle with two heads. In the real world, "Brahman" are a kind of cow named after the priestly caste of India. When the bombs fell, cows mutated and grew two heads, and were given the name Brahmin. They also have 8 stomach compartments, twice as many as normal cows.

Biology
The brahmin are quite hardy. They are a 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.

Though harder to kill with bullets than people, one dart from the Dart Gun kills a Brahmin. NPCs sometimes tell the player character that occasionally, a brahmin is born with a single head - essentially, a regular cow. These single-headed brahmin are ironically known as "mutated" brahmin.

All brahmins for some reason take less damage if you shoot them in the head than they do if you shoot them anywhere else.

Mad Brahmin
"Mad Brahmin" are believed to be either a Brahmin with a pre-war disease known as "Mad Cow Disease", or simply a Brahmin that has suffered brain degeneration to the point of hostility. Little is known about the condition, or why Brahmin would become aggressive. They are sometimes sighted among inexplicably rotting corpses of other Brahmin, suggesting some form of disease in the herd. Mad Brahmin are aggressively hostile except to those with the Animal Friend perk. Encounters of Mad Brahmin have commonly been reported near the entrance to Vault 101, Jury Street Metro Station, the Scrapyard, and Farragut West Metro Station, though Mad Brahmin could possibly be found all over the Capital Wasteland as a random encounter. One player even encountered a herd of Mad Brahmin at Robco Facility, and another near Smith Casey's Garage at the military checkpoint north of Smith Casey's. Though Mad Brahmin may be hostile, they are not deadly. Mad Brahmin can be outrun and due to their two heads, are prone to headshots, as well as being tipped and beaten with choice melee weapons.
 * If killed, they give 1 xp and you get Brahmin Steak from the corpse.
 * Regular Brahmin (i.e. not pack Brahmin) can be pushed over by crouching next to them and pressing the action button (only works if weapon is holstered). [Try it in Megaton on the Brahmin near the unexploded atomic bomb.]
 * If you have the Animal Friend perk, Mad Brahmin are not hostile, and can also be tipped over.

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 and 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. Brahmin hair can be woven into bags and ropes. 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, it has been said to help heal radiation poisoning. 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 hide can be used to make tents (or teepees), clothing, belts, saddle bags, shoes, leather armor, leather jackets, or a bizarre brahmin-looking disguise.

There are domesticated brahmin and wild brahmin. Wild brahmin can forage for themselves, and 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 - with strong exception to the deathclaws, which prey upon them among countless other, less powerful creatures.

The highly addictive drug Jet is synthesized from the fumes of brahmin dung.

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".

Brahmin tipping
In Fallout 3, you can go "brahmin-tipping" by crouching and activating any Brahmin that isn't attacking you. There's no activation symbol, like when you target a door and it says "open door", but you push 'em over all the same. It uses a unique animation seen at no other point in the game, and the Brahmin moos and turns into a ragdoll for a few seconds. This is an Easter egg, likely a nod to a quote from Tandi in Fallout, where she describes what there is to do in Shady Sands. She says that "brahmin-tipping gets old real quick."

In the Fallout demo, brahmin would occasionally be heard to exclaim "Moo, I say!" Additionally, Moira Brown's terminal refers to the brahmin language, raising the possibility that they are smarter than they appear. It is possible that brahmin actually say the word "moo" rather than lowing like cattle.

Brahmin-tipping also features as a special random encounter in Fallout Tactics.

Reference to Hinduism
Brahmins may be a reference to the Hindu culture. Hindus main God is known as Brahma, and the highest class of their society, brahmins, are a type of priest or teacher in their society. Another thing to note is that many Hindu gods have an extra limb or body part, and people or creatures born with such bodily features are looked upon as sacred and holy, often being reincarnations of a Hindu god. And of course there are cows, apparently Brahmin's prewar predecessors, which are also extremely sacred in Hindu tradition. This was the reason Fallout 3 was banned in India as to not cause offence to Hindus and those of the Brahmin caste.