FoE (c) Kkat
The bottlecaps for Sparkle Cola.
Sparkle-Cola was the most popular beverage in Equestria prior to and during the Great War. A carbonated soft drink, traditional Sparkle-Cola is a carrot flavored beverage similar to soda. After the war, Sparkle-Cola has remained a popular beverage and its bottlecaps are used almost exclusively as the de-facto currency of the Equestrian Wasteland. Backed by the value of water, merchants across the Wasteland supported bottle caps because the technology to manufacture them and paint their surfaces had been mostly lost in the Great War, which limited any counterfeiting efforts. Secondly, there are a limited number of bottle caps, which preserve their value against inflation to some degree.
PERSONAL HEADCANON (Applies to both FoE and FO) : Bottlecaps as currency was a result of not only those factors, but a marketing campaign by the Sparkle Cola company to boost sales; the Sparkle Cola Club, which was a company sponsored club dedicated to Sparkle Cola, which could be easily set up in schools, making it both lucrative and easy to afford. One of the main features of the S.C.C was its prize system, designed to perpetuate the sales of Sparkle Cola; it was a series of prizes that could be exchanged for bottlecaps, most of them cheap and worth less than the Cola itself, but foals didn't care, it became a huge success. Although some people protested the price, the range expanded far beyond simple toys, and as a result, after the war, ponies got the idea that bottlecaps had value in exchange for goods.