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. These brahmin explode on contact, making them very dangerous to unprepared players,especially at low levels. It's good idea to enter combat mode and run for the exit grid. But in Fallout 3 the Brahmin don't explode, they just attack.

This is probably a reference to mad-cow disease.