Quiz

What do a tram and a horse have in common, and what do a bus and an ox have in common?

In other words, what do the pairs (tram + horse) and (bus + ox) have in common? Or what do a tram, a horse, a bus, and an ox have in common? Or is the answer made up of two concepts: one is what a tram and a horse have in common, and the other is what a bus and an ox have in common?