Because it's half height, like a pony is to a horse.

My interpretation? it's the height of a hitching post where you tie up your horse (or pony) :)

Ah, both plausible and I'd have not thought of either. Thanks.

I thought it was similar to a dutch door where the top opens like a window but the bottom can be shiut to keep the livestock and the ponies out :)

I don't know if beer should be used as a comparison, but my husband said when he was younger that a regular beer was 12 oz and a pony beer was half as much - 6 oz..

Also, kegs of beer...and pony kegs (1/2 of a keg).

Mmmm. Beer. Thanks for the reminder!

My wife isn't half my height (6'4 versus 5'1). But she is half my weight. I sort of have a pony wife!! Rut row...

Rut row is right, Mongoct! DH teased our first 2 children when #3 was on the way. He kept saying he wanted a pony. They were 2yo and 4yo. and probably a bit confused.

In 1863, Walter Clydell of Leehigh, Nebraska created a short walled
stable for his horses. Soon after, as the wall gained popularity amongst
his fellow farmers, the public began to refer to this specific wall
height as a "pony wall".
-- https://en.wikipedia.org/wiki/Pony_wall

Lol, so there you have it @olychick!

Finally, enduring! lol