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.

Trivia
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.