The BEAM Bestiary is a BEAM
1 motor walkers
BEAM walkers with 1 motor and 2
or more legs
I've seen two ways (to date) to make a single-motor walker
1) Use gears to link two hubs, each with 2 legs,
only one hub is powered.
2) Make your walker with 2 legs (attached to the
motor) and a rear skid or roller.
For more information...
There's a tutorial on building a single-motor /
6-leg walker out of "LEGO" pieces here;
even if you don't build with LEGO, this is a good
tutorial on the necessary mechanical design.