Mad Brahmins

Mad Brahmin is a special encounter in Fallout 2 and Fallout 3 wherein you meet a herd of mad brahmin that come running towards you. In Fallout 2, these brahmin explode on contact, making them very dangerous to unprepared players, especially at low levels. It's a good idea to enter combat mode and run for the exit grid. In Fallout 3 the Brahmin don't explode, they just attack.

This is probably a reference to mad-cow disease.