The first trackball apparently used an actual bowling ball as its ball:

Like many of the world’s great inventions, the trackball as a concept was invented by a British engineer, Ralph Benjamin. Kept as a military secret for a few years, it was only fully implemented as a usable device by the Canadian navy some years later in its Digital Automated Tracking and Radar system (DATAR).

Nicknamed a “rollerball” back then, the trackball device actually used a standard Canadian five-pin bowling ball, mounted on several air rollers. A disk was mounted on each roller, which were hooked up with electrical contacts. As each disc spun in relation to the movement of the ball, “clicks” could be measured.