Referred to in the books as "Killing Devices", this extract is the first appearance.
"The long, black, curved shape that served as a head swivelled back and forth on the dull metal body without exposing a neck. The eyes were set deep in the metal, if those dark pits were eyes. The limbs seemed formed of large metal-coated bones - giant's bones? - and fine metal chains and rods that acted like muscles. Pulleys served as joints. There were three joints in each limb between the splays of knife-tipped digits on its feet and hands and the limb's connection to the body. That gave the thing two extra elbows and two extra knees. Its slender tail coiled and whipped, snakelike; it was tipped with a ball of spikes. The whole construction was nearly seven feet tall." (Pierce, 352:2001)
I chose this because I like the idea of the challenge of creating what is effectively a fantasy robot, and also rigging something that doesn't match the conventional body plan.
Pierce, T (2001) Squire: Protector of the Small 3 Reading: Cox and Wyman Ltd.