The nightkin should do a bit of a monologue revealing his hatred of the brahmin and then attack one of them. If you wait around until roughly midnight and keep an eye on the brahmin pen, you should be able to see a large Nightkin run out across the road and into their pin. They should also offer a small reward if you can manage to save the rest of their herd from whatever has been killing them. They’ll confirm the killings and note that every night at midnight they hear gunfire and screaming. You can recognize it because of the small pin with three brahmin next to their house. Dusty and Alice McBride live apart from the rest of Novac’s population, in a small house in the middle of town. You can also talk directly to the McBrides.
Obviously No-Bark’s a little off, but if you pay attention to his story it seems clear that a deranged individual with a stealth boy has been causing all of the damage. If you talk to No-Bark, you can learn that a machine gun wielding “chupacabra” has been killing the McBride’s Brahmin. It is quite possible that you won’t be able to properly finish it with a fair bit of luck from the random number gods.
Unfortunately this unmarked quest seems to have some really buggy scripts.
HOW TO KILL NIGHTKIN FREE
It’s also good for easy experience and some free brahmin steaks. It’s a fairly useful quest, since it can help raise your reputation in Novac quickly and help net you a free room at the large motel. The Screams of the Brahmin - Introduction