A Trackball is a feature in the Subspace Emissary. Trackballs first appear in The Jungle as pink circles attached to silver pathways. Attacking the circle causes it to zoom along the path, damaging and often KO'ing any enemies it meets. The strength of the original attack 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). If the path is closed, the Trackball will remain where it ends up once it stops moving. Trackballs cannot be hit until they stop moving.
Trackballs generally appear in two cases: areas with lots of enemies, and areas with certain blocks that only Trackballs can break.