When the player character approaches the cattle pen at Brooks tumbleweed ranch a nightkin will uncloak itself attempting to sell "wind-brahmin" to the Courier. His fee is all of the player character's caps and if the tumbleweeds are not purchased, he will become hostile.
Originally, choosing to buy one without asking how much resulted in the nightkin giving the Courier a Stealth Boy for free. With patch 188.8.131.525, the nightkin no longer gives a Stealth Boy. After the patch, agreeing to buy a "wind-brahmin" without asking how much results in the nightkin taking all of the player character's caps instead.
Further interactions, such as suggesting that the tumbleweeds are not in fact "doggies," will also result in the nightkin becoming hostile.
The nightkin will retain the caps taken in their inventory which become accessible upon death. Their Stealth Boy can also be taken at this point.
The nightkin trying to sell you the "wind-brahmin" may not show up, but in an entirely different location (i.e. the Strip) and try to sell it to you. He can show up in any area of the game world later on, including DLC-only areas. When he appears to you outside of Brooks tumbleweed ranch, there is a good likelihood that he will be hostile to non-player characters around him and club them to death before initiating his odd dialogue with the Courier. If you load the game, it seems that he may not appear again, which means you will probably have to deal with him elsewhere. [verified]
This may also happen on PC. In fact he may even skip the dialog and suddenly appear right in front of you yelling: "I'll kill you!". This may also happen inside of a building. (e.g. one of your safehouses.)
If you kill the nightkin and come back a few days later, upon entering the pen you will hear the nightkin coming out of stealth but nothing will appear anywhere.[verified]
When initiating dialogue, if you accept the offer without asking how much, he will give you the Stealth Boy without taking any caps.[verified]