When the player 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's caps, if they are not purchased he will become hostile. Originally, choosing to buy one without asking how much resulted in him giving the Courier a Stealth Boy for free. With the game patch released in July 7–8, 2011, the nightkin no longer gives a Stealth Boy. After the patch, agreeing to buy a "wind-brahmin" without asking how much results in him taking all of the player's caps instead.
PCPlaystation 3Xbox 360 The nightkin trying to sell you the "wind-brahmin" may not show up, but in a 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 character's 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.)