Money in Equestria is interesting. As of the the most recent episode, all of Equestria accepts the bit as the universal mode of trade, whereas most human countries will make a new currency when they are even slightly different. But its interesting to ask where the bit came from, since we see its use since the old times from ponies like Granny Smith.

My suspicion is that the bit is a fiat currency, created to keep all the races of Equestria equal and happy. Let me explain. The way gems exist in Equestria, I suspect they were originally used as a manner of trade, just like humans did with gold. Its hard to find, nice to look at, and is a lot more useable than the barter system, even though that still exists in Equestria. However, when things become money, people will try to circumvent the system in order to make themselves a whole lot wealthier. Since technically the more you find of it, the more powerful you become since you can obtain more things. This is where I find the problem. Unicorns and Alicorns, with their active usage of magic, would be able to identify and collect gems quickly, making them the wealthiest of the bunch. This supports their original ideology as a aristocracy, shown in the founding of Equestria, since they did technically have a lot of the money.

Before you ask about why they fought over food just before the founding of Equestria, the answer is simple. Money breaks down in times of crisis, since supplies have to be rationed. Thus, it didnt matter when the Windigoes came, everyone was on their own when it came to food and survival, money wasnt a question. It could also explain why they were, I quote, "Starving and freezing" in contrast to the Earth Ponies and the Pegasi, which only had one of said problems each. Maybe they over-relied on their stocks of gems, that they kept no actual stocks of firewood and food. Also addressing the potential barter system the Earth Ponies had in order for safe weather and the Sun and the Moon cycles from the Pegasi and Unicorns respectively, it comes back to the valuation of goods and services. How do you value one day of clear skies in terms of food? And what about the fact that you need good weather to produce food to begin with? Some form of credit must have existed, promising a certain amount of food in exchange for perfect weather for the year. This would have been the foundations of a currency, the promissory note.

Back to the explaination. The Earth ponies would not need to care about finding the gems, since they would simply be able to produce the goods that the rest of the ponies needed. The unicorns would have to use the gems to trade for food and materials that the Earth Ponies are so adept at obtaining. This explains the fact that a lot of Earth Ponies have professions in trading and selling, considering that is the skill they would have needed to get the best value out of their goods, ingrained in their culture from the beginning. They would then use these gems as payment for good weather and the shifts in night and day.

The Pegasi would need to focus completely on controlling weather. Any kind of mess-up on their part would result in failed crops, and tension between them and the Earth Ponies. This could be behind the reason all Pegasi value their physical strength so much, since they needed a lot of workers to manage their vital service, which is good weather for the Earth ponies. They would have been paid in return in the crops that were produced. This single-industry centric culture would thus have been created, like the Weather Factory in Cloudsdale, and the fact that they conscript all Pegasi to help in major weather events, like Winter Wrap Up and resupplying the Weather Factory with water, as per Hurricane Fluttershy.

This kind of system would have worked fine, except that ponies wouldnt be too happy with the Unicorns, who take on an Aristocratic role and just simply raise the sun and moon, but otherwise just use their gems to get everything else they need. This would carry on past the founding of Equestria, until someone can make a change in this original system. Enter the Alicorns. Since they represent all races, they can make changes without causing undue harm. Thus, one thing they might have done is to make a fiat currency to replace gems, since nobody had anything to lose from the switch. The Unicorns are fine being there, since they would have a lot of wealth through their gems. Many years of trading through gems means they could have built up massive cities and massed stockpiles everything, so losing the gems wouldnt be that harmful. The Earth Poines and Pegasi wouldnt be unduly upset, since they now would have more control over their currency instead of it being managed by a single race.

And thats my headcanon on money in Equestria.