In the song "Jackson" sung by Johnny Cash and June Carter, June is dancing on a pony keg What is a pony keg?

unlike a regular keg, a pony keg only holds 5 gallons of beer

1/4 or 1/2 keg of beer

1 pony keg = 29.3369415 liters

The Pony Keg

A pony keg is a beer vessel containing 7.75 U.S. gallons (29.33 liters) of fluid. It is half the size of the standard beer keg and equivalent to a quarter of a barrel. It will serve roughly 82 twelve ounce cups. The shape of a pony keg resembles that of a full-sized keg, while being much easier to move manually. However, it shares many inconveniences with the bigger kegs, such as: often requiring a deposit when purchased; needing a tap to serve the contents; and posing difficulties in determining the amount of beer remaining (weighing the keg or observing its buoyancy are common techniques). Despite this, a pony keg can be a cheap way to provide beer for a medium-sized party.
VID
http://youtube.com/watch?v=a3j016ou-jQ

I would guess a smaller keg of beer. You know like a pony bottle of beer is smaller. You can still buy them at the package store.

A pony keg is just a reference to a smaller than normal size keg of beer. Now days they are called party kegs because they hold less and the full size kegs are more expensive.

A small keg of beer..