Artist's commentary
From what I hear, the kappa was dragged all the way to the stable, where it asked, "What will you do? Let me go? Kill me?" It was made to promise that it would never do wrong to horses again and then set free.
It's that kind of story.
Nitori seems like she'd use the power of science to drag it in, though. :-)
Incidentally, they also do kappa fishing here, in which they hang cucumbers from the ends of fishing devices.