...perhaps we're disagreeing only over semantics.
A bottle that you take down with you intending to breath while you're down there; e.g. the gas in that bottle is planned to be used, is either a stage bottle or a deco bottle. A stage allows longer bottom times, longer penetration, etc. A deco bottle holds different gas mixes designed to efficiently promote offgassing of nitrogen and, to a lesser extent, helium.
A bottle that you take down with you that you ONLY intend to breath if you suffer some sort of catastrophic failure of your primary backgas; e.g. its a completely redundant gas source for emergency use only, is a bail-out bottle.
The term "pony bottle" means so many different things to so many different divers that it has no one single, agree-upon definition - the term basically is meaningless.
So, whether he wants to take down a stage bottle, a deco bottle, or a bail-out bottle; it would be good to have this defined beforehand - particularly if he's going to put EAN40 into the thing, then try to go to it if he gets into an OOA situation at 110 fsw.
The point is that doing gas switches to different gas mixes at depth is a good way to die, if you don't know the MOD for the gas you're carrying and you don't know a reliable way to ensure you're switching to the right tank. More training would likely be a good thing before one begins carrying down slung bottles with EAN mixes, whatever you choose to call them and for whatever purpose.
And thats not garbage...
