Electric Clown Fence
From the Super Mario Wiki, the Mario encyclopedia
An Electric Clown Fence is an impediment in Wario World. It appears three times in Wonky Circus, serving as a path-blocking enemy that parallels those found in previous and later worlds. It is a target game of sorts. Wario must pick up an enemy (usually a Clown) and try to hit one of the moving targets on the fence. Wario must hit the target twice more to destroy it, then do the same with the others. However, each time Wario hits a target, it will get smaller and faster, making it harder to hit.
It's also possible to simply take out the target with one Mega Toss.