Pony Bottle

A small independent cylinder with its own regulator carried as a fully redundant emergency gas supply.

A pony bottle is a small extra tank, commonly 2 to 6 liters (13 to 40 cubic feet), carried with its own dedicated regulator as a self-contained backup gas supply. Unlike an octopus, which still depends on your main tank, a pony is fully independent: if your primary supply fails outright, you switch to the pony and own enough gas to make a controlled ascent with a safety stop.

Sizing follows from depth and breathing rate. A stressed diver at 30 m breathes several times their surface rate, so honest rock bottom style math usually points to at least a 3 liter (19 cubic foot) pony for recreational depths, and larger for deeper profiles. The bottle is slung at your side or mounted to the main tank, with the regulator stowed where you can deploy it without looking.

Ponies are popular with solo divers, photographers who drift away from buddies, and anyone who wants redundancy without doubles. Analyze and pressure check it before every dive, and practice switching to it until the motion is automatic.