A pony tank is a smaller tank, which is carried additionally to the mostly single main tank. It possesses its own regulator with first and second stage and sometimes a gauge too. It will be mounted either at ones side or in front of the body or fixed at the main tank.
A pony bottle scuba tank is the only true redundancy you can rely on.

When shipped the tanks valves will be removed and the tank will require re-inspection before being filled.
All Catalina cylinders are Nitrox Ready!