A trackball
Trackballs generally appear in two cases: to help in areas with many 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 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.
A trackball and trackblocks.