Wind-Brahmin Wrangler

"They are very good Wind-Brahmin. All Caps."

 is an unmarked quest in Fallout: New Vegas.

Quick walkthrough
Either buy a "Wind Brahmin" for all your caps or refuse and risk a fight with the nightkin.

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. If you choose to buy one without asking how much, he gives you a stealth boy for free. A stealth boy can be pickpocketed from him, even if you received a stealth boy from him beforehand. 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 dropping some caps before the transaction for a "bargain" Wind-Brahmin and a Stealth Boy.

Bugs

 * Sometimes, the Nightkin will not spawn in his specified location. This usually occurs when the player encounters him and then loads a previous save. When the player either signs off or restarts their console, the Nightkin will spawn at whichever location the player happens to be. He will spawn and talk to the player normally, as if he was at Brook's Tumbleweed Ranch. Either before or after talking to the player, he will then attack all other friendlies or hostiles in the area (except other Nightkin). This can be either dangerous or helpful, depending on the player's current location. For example, spawning him in a friendly area might not be the best idea, but spawn him near a group of unfriendly raiders and he will either soften them up or completely eliminate them for you, like a one-time-use companion. This bug can be repeated indefinitely until the player saves after he is spawned.
 * Sometimes the Nightkin might not spawn in his scripted location, but will instead be at Camp McCarran, The Fort or any other faction locations.