Wind-Brahmin Wrangler

Wind-Brahmin Wrangler or Tumbleweed Rancher (GECK name is an unmarked quest in Fallout: New Vegas.

Detailed walkthrough
Approaching the cattle pen at Brooks Tumbleweed Ranch will uncloak a nightkin selling "wind-brahmin"(also known as tumbleweeds). His fee is all of your caps. If you don't buy them, he will become hostile. Originally, if you choose to buy one without asking how much, he gives you a Stealth Boy for free. But with the game patch released July 7–8, 2011, the nightkin will no longer give you a Stealth Boy. If you agree without asking how much, he will take all of your caps instead. A Stealth Boy can be pickpocketed from him. Further interactions (insinuating that the tumbleweeds are not in fact "doggies") results in the nightkin becoming hostile towards you.

If you buy a wind-brahmin, the nightkin can be killed for your caps. Just in front of the pen is a dead wastelander. If killing the nightkin is too much trouble, it may be worth using the dead wastelander to briefly 'hold onto' most of your caps before making the transaction for a "bargain" Wind-Brahmin.

Bugs

 * 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 Brook's 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.
 * 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. (i.e. one of your safehouses.)