A Trackball is a fixed item in the Subspace Emissary. Trackballs first appear in The Jungle as pink spheres attached to silver pathways. Attacking the circle causes it to speed along the path, damaging and often KO'ing any enemies it hits. The strength of the attack used to hit it affects how fast and how far the Trackball moves, as well as how much damage it does. If a Trackball's track is "open" (that is, it has a start and an end), the Trackball will return to its starting point once it reaches the end or runs out of momentum (doing no damage on the way back). On the other hand, if the path is a closed circuit, the Trackball will remain where it stops once it loses all momentum. Trackballs cannot be hit while in motion.
Trackballs generally appear in two cases: to help in areas with lots of enemies, or to unblock paths with certain blocks that only Trackballs can break (called Trackblocks). Trackblocks will be destroyed no matter how fast a Trackball is moving (as long as it's not in reverse).
While Trackballs are generally activated by attacks, there are areas (for example in The Great Maze) that feature "chains" of Trackballs. When hit, the first Trackball will smash into the other Trackballs, which causes them to move at their greatest possible speed (regardless of how fast the original was moving).
There are a few Giant Trackballs in The Wilds II. Their paths remain invisible until they are struck, and obviously they deal a ton more damage.
On higher difficulties, Trackballs are harder to move, and run out of momentum faster, meaning they must be hit stronger and more frequently to destroy enemies.