The Zvex Woolly Mammoth is a Bass fuzz pedal designed by the boutique virtuoso Zackary Vex. It's been in his product line for many years and has proven a popular effect for both guitarists and bass players alike.
Here's the description from the Zvex website:
"Sensitive and touchy, This bass fuzz has tremendous bottom end and a beautiful harmonic structure that sounds blistering with a guitar too..
TAMING THE BEAST
The harmonic structure can be radically altered using the "PINCH" knob. It adjusts the pulse width of the waveform.
In the leftmost position, the waveshape is quite symmetrical which produces a smooth creamy sound. Turning it up slightly introduces lovely intermodulation distortion. Turning it clockwise narrows the waveshape into asymmetrical pulses, making the tone reedier and brassier. This also introduces an unusually smooth 'gating' action, which gives you an absolutely eerie silence between notes.
At most settings you will notice that low frequency pressure is preserved. This circuit has a frequency response that continues several octaves below audibility. The subs will knock everything off of your amps, so put the the bottles on the floor. No kidding.
The "WOOL" knob adjusts the amount of fur around the note. The "EQ" knob tilts the spectral response, turning to the left adds lows and softens the highs. Turning to the right adds highs, then mids, and finally reduces lows. the "OUT" knob Is designed for smooth repeatable level settings
In our continuing effort to be friendly towards the earth, the universe and it's inhabitants; no animals were harmed during the making of this pedal.
Oh, and the circuit (designed by Chuck Zwicky) draws less than one-half of a milliampere (0.0004 amps, typical). That is less than one-hundredth the current consumption of most 'commercial' pedals, giving this pedal about as high a battery life as you can find anywhere. In fact, if you use an alkaline battery and always unplug the input cord between sessions, you may never have to change it at all!"
And a demo video of the Woolly Mammoth in action:
The schematic has been about for many years now (I believe it was one of the first on freestompboxes), here it is, traced by AnalogGuru:
As you can see it's the standard Fuzz Face layout with a few mods here and there. The large input/output caps allow more bass through, there's a Big Muff style tone control at the end of the circuit and the pinch control, which controls feedback between the two transistor gain stages.
Here's an uncredited but verified vero layout for you:
And the freestompboxes.org thread (all 26 pages of it!) for reference: http://freestompboxes.org/viewtopic.php?f=7&t=4