It is likely that brahmin developed from certain pre-War bovine species. Radiation from nuclear fallout has caused multiple genetic mutations, such as the growth of several disfigured horns, disfiguration of the skin, udders and most notably the growth of a second head.
Brahmin and radstags are the only creatures in the Commonwealth to develop a second head due to their exposure to vast amounts of radiation. Both species are mammals and quadrupeds of large size.
Brahmin, being important in the development of settlements, have a more active role in Fallout 4 than in previous games. A feed trough can be built to attract brahmin to the area and brahmin caravans are used by settlers (assigned by the player character) to provide supply lines between various settlements. They drop fertilizer that can be used for growing crops and is an ingredient for jet, fireworks and explosives.
One of the objects that can be crafted and added to a settlement is the heads of a brahmin mounted on a plaque.
Kelly will sell a brahmin to the player character for 100 caps. The player character will need to choose a settlement to send the brahmin to. This can be done multiple times.
Brahmin in some occasions may end up in a player character's built house or in a pre-existing house. This can be problematic as when they do get inside they can block doorways and entry.
Some settlement brahmin drop a synth component upon death. This could mean that, like some settlers, the Institute created synth brahmin to infiltrate settlements. Since the Institute has a history of creating synth animals this could be a way for them to observe how well they pass for real brahmin.
If the player character attacks a brahmin in one of their settlements, the settlers will turn aggressive against them. Going away and returning after 24 hours will make the settlers become passive again.
It is unclear how brahmin reproduce as they all seem to have udders, which suggests they are all females.
It's possible that all brahmin, including males, have udders, another possibility is that brahmin are hermaphroditic.