So let me preface this by acknowledging that it's pretty fanciful, but:

What if you've got some pangolin-like species that rolls into a ball for defense.  From there it develops a technique to roll downhill into a prey animal.  This technique not only surprises the prey but gives it a heck of a wallop to boot.  One downside is that it's impossible to see when you're rolled into a ball, which makes aiming difficult.  The prey has a good chance of dodging this attack, increasingly so once they start evolving to avoid it.

Eventually, mated pairs of these pangoloids develop a behavior in which they grasp hands before rolling up, so that they make a dumbbell shape--two spheres connected with a bar between them.  The increase in mass, along with the inherent advantage of numbers, allows these pairs to bring down bigger targets.  One disadvantage is that, if both parents hunt, there's no one's guarding the young some of the time.

Flash forward a few million years.  Consider a mated pair of pangoloids and their baby pangoloid.  They're all out hunting together.  The parents hold hands and then roll into balls.  The juvenile grasps the parents' arms with his specially greased paws and hangs down from them.

The parents start rolling downhill toward their prey.  The juvenile, along for the ride, keeps an eye on the prey and communicates its current position to his parents, either through touch or through sound.  The parents adjust their angle of attack accordingly.  The end result is significantly increased accuracy, which results in increased survivability.

Maybe from there, the child evolves the ability to act as an engine, by actively running instead of just passively hanging on.  This would allow the attack to work with shallower slopes or even flat terrain.

Like I say, pretty fanciful.