When discussing alternate air sources and configurations, back when the concept of an safety second as a normal gear configuration was new. PADI Divers Manual rev1984. p58:
A redundant system,usually smaller than the primary system, consists of a totally separate air supply and demand regulator. Frequently a small scuba tank with a standard regulator is attached to the side of the primary scuba unit and is referred to as a "Pony Bottle".
There is a history of the Pony Bottle being an emergency alternate air supply, regardless of size.
Bob
A separate tank and reg is IMHO currently known as a bailout. This term really came mainstream with the advent of rebreathers which required an open circuit bailout. The term "pony" I have always taken to be a US phrase derived from "pony engine" (in English a "donkey" engine) - a small engine used to start a bigger one.
I think it is important to define what is being discussed if you are to have a benefit from the discussion. So, I query what "pony tank" means to the person using the term. Here in the UK the vast majority of divers would understand it to be a back mounted tank of about 3L (6CF) capacity. There is no "agreed" rigging and regulator configuration. Hence you can dive with a number of pony equipped buddies all of whom will have different regulator set ups and their own personal idea about what to do in an OOG situation. Their own theories about AAS and what to donate, how to deal with a regulator being taken by a panicked diver and so on.
Like many people I have an issue with this particular arrangement, not an issue with the idea or practice of using a bailout. Sadly this often leads to an argument about pony tanks (a popular argument on the Internet it seems) where either side is actually arguing about something different.
A bailout tank or redundant supply is pretty much a mandatory item for solo diving. But when solo there is only the diver him or herself and the bailout is for the diver should the primary gas source fail. Whatever regulator configuration you use is your own choice and will be - by definition - the one you prefer. The OP opens the subject of routine use of a bailout for buddy diving. This adds the dimension of how you interact with your buddy if there is an OOG incident. At the least a very thorough pre-dive briefing is required and the two divers must agree the procedure and commit to it before the dive takes place. This is not a practice I witness very often. Equally both divers must be sufficiently able that in an OOG situation they have the discipline to follow the agreed procedure and commence breathing from the appropriate regulator and begin the ascent. This is an assumption whereas the buddy diver will have been trained to make an ascent with an octopus during their basic training.
Of the two divers the one equipped with a bailout has the least motivation to maintain buddy contact. S/he is self reliant, a solo diver. The buddy must either trust this person or take the logical step of equipping themselves with a bailout as well. Eventually this will lead to everyone being bailout equipped and everyone "diving solo".
Again and again I read that the "instabuddy" is the issue. Quite reasonably people say they will not trust their life to a person they have just met. (I assume these people never fly an airplane, ride a train or take a bus?) My take on that is simple - the guy with the pony is a solo diver (as am I) and I am not diving with them. I'm just fine on my own thanks.
