Here's a version: sometimes horses are fitted with blinkers. As a result, their lateral movement is greatly restricted by the rider's will, and they become less skittish and more predictable. In general, I would say that the concept of a horse with blinkers is similar to the concept of a tram.
A bus and an ox, on the other hand, have almost no restrictions on lateral movement.
But that's just a bit of a stretch. Ideally, there should be an option to answer in a way that immediately makes it clear which pairs are suitable and which do not illustrate the problem.