I’m pretty sure the answer is balding hair.
While both have to very different meanings, both have a definition of meaning something small (alley - small passage, pony - small horse, pour, or amount of money). Both are 6 letters, but when made plural, are spelling differently (alleys - just add s, ponies - drop the y, add ies). Both are Latin in origin, changing letter to French. But alley is late middle English, with pony being from the mid 17th century.
Conclusion summarizes the main point i think
