Brahmin Herd (Fallout)

Brahmin Herd is a special encounter in Fallout.

For this encounter, you come across a herd of wild five Brahmin. They say "Moo!" or "Moo, I say." without attacking the player and leave as if nothing had happened.

This special encounter is really hard to get and more rare than others, even with 10 Luck.

Behind the scenes
A similar special encounter appears in fallout 2 is called Mad Brahmins. It's exactly the same except that after a while, instead of starting, the Brahmin explode.