There’s plenty of disagreement among fanfic writers in the My Little Pony: Friendship is Magic universe as to how much a bit “really” is. Better we should ask the basis for those bits, inasmuch as our own humanoid currency is sort of questionable these days:
[W]hen you hold cash, it’s supposed to lead back into something tangible. Whatever someone has assigned value to and can’t readily be carried, portable money substitutes for. Precious metals, for whatever reason someone decided they were worth something. Jewels, perhaps. (Humans have a certain weakness for shiny objects. Ravens with slightly improved grasping digits and lower impulse control.) You can’t ask the government for that backing material any more, at least locally. There are still silver certificate bills in circulation — they were supposed to be pulled, but collectors and dusty rainy-day stashes occasionally release a bill or two — but unless they’re crisp enough to resell as that collectible, they can only be used for their face value. The value we’re all basically lying to each other about because as long as we all lie, the system remains more or less intact.
Once upon a time, the United States ran on silver. Then gold. (Today, possibly debt.) The currency must lead back to something, even if that thing doesn’t exist.
And what might that thing be in Equestria? Think power. Herewith, a possible basis:
If the name “bits” hadn’t been assigned to us, I would count Equestrian currency in sols (or lunes). Because there are times when currency leads back to labor. You clear my fields for eight hours and I’ll give you four chickens. However, I really don’t feel like keeping those clucking menaces around, so here’s a piece of paper which you can present to a man in town, and he’ll give you the chickens for me … on such small things do economies grow.
And the ultimate labor is that which makes Sun and Moon keep working.
What’s Equestria’s ultimate promise to the world? We will keep the cycle going. And that’s about as strong a backing for a monetary system as you can ask for.
So if I was working on this, I would have the money powered by pony labor, with the sisters at the top of that scale. Ultimately, money is traded out to the other nations with the understanding that the palace will maintain the orbits. Oh, there are other forms of ponyhours being traded out: send us these goods and we’ll dispatch pegasi to adjust your weather system. Access to magic — especially that which the other species don’t have — gives a nation some powerful leverage in the world economy, although some of that is countered by what said other species can bring to the table. But at the far end of the chain … there is a simple promise. What backs Equestria’s economy is the most fundamental labor to exist in that world, performed four times per cycle — or there is no cycle at all.
If this doesn’t make sense, imagine trying to explain currencies in this world, most of which have value only because large institutions with stores of arms say they do.