I was listening to the song "Jackson" written by Jerry Leiber and Billy Edd Wheeler, and sung by Johnny Cash and June Carter. Near the end there's a stanza that refers to a "pony keg":

But they'll laugh at you in Jackson, and I'll be dancin' on a pony keg. They'll lead you 'round town like a scalded hound,
With your tail tucked between your legs,
Yeah, go to Jackson, you big-talkin' man.
And I'll be waitin' in Jackson, behind my Japan fan.

I confess that before this song, I'd never even heard of a pony keg. It is in fact a beer keg, one that's half the size of a regular keg, and holds 1/4 of a barrel, or about 29 liters, or 7.75 gallons of beer. That's enough for 62 pint glasses of beer. I note that this is not the same as the "refrigerator kegs" made by Heineken and others.

I have been unable to find a legitimate etymology for the phrase, though slang dictionaries and the Internet are rife with imaginative folk etymologies. I suspect the derivation has something to do with the idea that a pony is a small horse, much like a pony keg is a small keg. Pony, etymologically speaking, dates to c. 1659 in English, or more specifically, in Scots. Pony is derived from French poulenet, "little foal," c. 1444, itself a diminutive of Old French poulain, "foal," from Latin pullanus, "young of an animal," a diminutive form of the Latin pullus, "animal young."

For the curious, I've embedded a video of Johnny Cash and June Carter singing "Jackson." They won a Grammy Award for Best Country and Western Performance for this song in 1968.