SCUBA diving I think. There are more than a few on here who like salt water so much that they get in it and swim around. Pretty cool!!
I still don't know why you would want your pony in there though. . . .
ok here is when u go scuba diving u have a main tank, ok, then as a precaution u have a smaller spare air tank, which is sometimes called a pony tank, that is usually attached to the main tank.
or for other air mixtures for deeper diving. TRIMIX. Cant remember its either Argon or Halon they use or 100% oxygen. There is another method to. Its called spare air. Do a search on it. They usualy are not that big though. Only like 2Cubic Feet. Might be cheeper than getting another bottle along with a new reg to fit it.