SF Klein
Senior Retro Guru
It's a single speed so not pure MTB and it's also not from the 1990's, but I like it anyway. I bet their fully geared version of this should still come under 20 pounds.
http://www.youtube.com/watch?v=wkUaRQDq ... r_embedded
http://www.youtube.com/watch?v=wkUaRQDq ... r_embedded