OK, I want to talk about terminology a little, and to do that, I want to start with describing a relatively serious tech dive.
The diver enters the water with 5 tanks: 1) 2 backmounted doubles with 10% oxygen and 70% helium (10/70), 2) one AL 80 with 21% oxygen and 35% helium (21/35), 3) an AL 40 with EANx 50, and 4) an AL 40 with 100% oxygen. On descent, the diver does not want to use the 10/70 tanks because the oxygen level is so low, so he breathes from the 21/35 until he reaches the depth of 190 feet, at which time he switches to the 10/70 tanks on his back. After completing his bottom time at about 300 feet, he ascends to 190 feet, at which point he switches to his 21/35 tank to begin his decompression stops. After about a dozen such stops, at 70 feet he switches to the 50% tank, and at 20 feet he switches to the 100% tank.
Horrors! He used the the same 21/35 tank for both his descent and his decompression! That means he used the same cylinder both as a stage bottle (or travel gas) and a decompression bottle! He used one bottle to serve two purposes! Shouldn't he have used two different tanks so he could have given them two different names? Well, as horrific as that sounds, it happens all the time.
So how does this relate to this thread?
People use pony bottles in recreational diving to give them a last resort source of gas that will get them to the surface in a dive that does not require anything more than a direct ascent to the surface. How much gas does that require? Not much. Actually, you don't truly need anything--a CESA will do the trick in a real emergency. If someone takes an AL 40 for a pony bottle, what is it other than the phrase "pony bottle" that prevents him from planning (key term) to use it during the dive for a short period of time (stage bottle!), leaving plenty of reserve for use in an emergency (pony bottle!)?