The 'Little Roaming Tractor', from the Zkwyx Meccanaut book. The tractor appeared to Zkwyx in a dream and he later described it to the engineers on his planet, Meccania, who constructed a working version.
The front wheels are fixed and don't touch the ground, the 3rd point of contact being provided by a wheel under the body, mounted on a near-vertical revolving shaft; the top of this shaft is journalled, off-centre, in another wheel, revolving at a different rate to the supporting wheel. This gives an apparent random path to the model. The rear wheels are not powered.