Pony bottle

Pony bottle

A pony bottle (left) attached to a larger cylinder
Uses	Scuba diving cylinder for supply of emergency breathing gas
Related items	Bailout bottle

A pony bottle or pony cylinder is a small diving cylinder which is fitted with an independent regulator, and is usually carried by a scuba diver as an auxiliary scuba set. In an emergency, such as depletion of the diver's main air supply, it can be used as an alternative 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 is a totally independent source of breathing gas for the diver.

Pony bottles are often used by divers who understand that no matter their preparation and planning, accidents may happen, and cannot, or do not choose to depend on another diver for emergency breathing gas. They are carried by the diver in one of several alternative configurations, and the capacity and contents should be sufficient to allow a safe ascent from any point in the planned dive profile. The name pony is due to the smaller size, often of only a few litres capacity. The term is generally used by recreational divers. Professional divers would normally refer to a cylinder in this service as a bailout cylinder or emergency gas supply.