High Quality Content by WIKIPEDIA articles! A pony bottle is a small independently filled diving cylinder, often of only a few litres capacity, which forms an extended scuba set and which is fitted with its own independent regulator. In an emergency, such as exhaustion of the diver`s main air supply, it can be used as an alternate air source or bailout bottle to allow a normal ascent in place of a controlled emergency swimming ascent. The key attribute of a pony bottle is that it provides a totally independent and redundant source of breathing gas for the diver.