There rare, I have the Mongoose NX series on my auto ebay search and I've seen like one in the last 5 years or so. I remember that style came out in 1998, and ran till 1999 and I think Mongoose got sold off to pacific cycles right after that. I think the frame was a bit on the heavy side and was a bit of a gimmick more then any kind of useful innovation. I don't think they sold to well at all but I could be wrong. That said I'm a huge mongoose fan so I think there pretty cool. Is it the yellow one? I think that was a "Xt" level bike, and the 9.1 was xtr, but you would have to source the catalog for more info. They had both a hardtail and FS in that style, I've been hoping to find a cheap hardtail to turn into a cruiser, but not much luck finding one. Post some pics if you have the chance.