This might help you out a bit, depending on where that ad was from. In my part of the world (northwestern Minnesota), the word "snowcat" just means snowmobile, like the little fast machines with skis and tracks that this page is about.
I think this is because the first snowmobile made by Polaris was called the Sno-Cat, but I'm really not sure.
So, if you found an ad from an individual in NW MN, that's probably what it is.
But, I looked on the internet and found that "snowcat" usually means a totally different machine. So if the ad were from some company or from a person not from NW MN, this could be what they mean by "snowcat"