Beer, Beer and More Beer—the best store, ever—is featuring this: the Tote-A-Keg. It's simple, yet amazing. This cooler will hold a 2.5 gallon pony keg. It uses CO2 to dispense the flowing beery goodness through an outer spout. The handle and wheels make this device easily the best accessory of any drinker. Take it to the BBQs this summer, tailgating or hell, just roll it along with your as you go from party to party on a Friday night. Unfortunately, the Tote-A-Keg totes a $150 price tag. Being an alcoholic is damn expensive, I should know, my Tote-A-Keg is on the way.
There is no such thing as a "Pony" keg. People use the word interchangably to describe anything smaller than a "normal" keg of beer (actually the "normal" keg is a half-barrel keg).
There are half barrels, quarter barrels (short and tall) and sixth barrels (short and tall). There are specialty sized kegs such as the so called 1Gal "Party Keg" and others. This thing holds a 2.5 gallon corny keg.
Unfortunately, you can't really buy any commercially brewed beer in this size keg, and if you are buying from a microbrewery, you will have to be lucky in order for them to either fill your own keg or fill their own kegs in this size. The usefulness of this thing is also somewhat dimished by the fact that it uses 12g CO2 cartridges to pressure the keg without the use of a regulator. Not only is this expensive and wasteful, it's also a bit dangerous.
Even though this cooler system is only useful to those who brew their own beer, I wouldn't even recommend it for that. If you want a nice portable system, opt for one that uses a real CO2 tank with a regulator (they have some that use paintball gun tanks) and opt for one that will hold common commercial sized kegs — at least short 1/4 or 1/6 barrels. Alternatively you can go for a system with a non-cooled keg and run the beer through a long chilling coil inside a cooler filled with an ice water bath. The bonus with that is that it works for any size keg, too. Just make sure you get through it in one day!