Wind-Brahmin

Wind-Brahmin are just essentially Tumbleweeds. However, in a few random encounters a Nightkin will approach you when you are at Brooks Tumbleweed Ranch, and ask if you would like to buy a "Wind-Brahmin". I'm not sure if its random but when the Nightkin asks if you want to buy "Wind-Brahmin", and you just say "Okay" he gives you a Stealth Boy and you don't loose any caps.