This small scuba tank, which provides a backup source of air, can be a life saving device. It resembles a fire extinguisher and contains a small supply of oxygen for divers. The larger versions are usually longer than 15 seconds and might last up to a minute or two.

Pony bottles are usually hooked to a diver's back, close to the main cylinder. They add to the overall weight; thus, divers are advised to check how these back-up devices affect their movement in water, especially in different depths.

This device is a safety measure in order to counter emergency situations. The probability of an out-of-air situation is low with equipment that undergo regular check-ups. However, some studies do point out that running out of air tends to be a somewhat frequent occurrence for the non-professional diver.

There are a number of benefits of carrying a pony bottle. Firstly, divers will always have a back-up plan on themselves. Usage of this device depends on your own capabilities as opposed to a human buddy. Human buddies who deal well with emergencies and/or are skilled at using equipment quickly, are of course a lifesaver. On the other hand, a buddy minus these features, would be less reliable than a pony bottle.

On the other hand, there is considerable debate around the usefulness of the pony bottle. Some people argue that smaller pony bottles contain so little air to be useless in a tight situation. Others point out that large pony bottles add to one's weight, bringing in a whole new range of annoyances.