Pleo's tail is what gives him the most personality. It's hard to understand without seeing him move. When you pick Pleo up by the surprisingly-durable tail, he detects the resistance and direction change and immediately starts squirming and squealing. When you put him back down, he quivers in shock and fear.
The tail is composed of 19 vertebra, 4 wires, 2 motors, 8 gears, 2 force feedback sensors, and a tremendous amount of ingenuity.