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Moesia was an ancient land comprising parts of present-day Bulgaria, Serbia, and Romania. It was probably the first Greek colony on Black Sea, with a dominant feature of the land being the mighty Danube River, at which delta the city of Istrus was founded by Milesean settlers in 7th century BC. 

Istrus and other major Black Sea coastal cities produced coinage employing the dramatic motif of an eagle perched atop the back of a dolphin, as shown on the reverse of this 4th century BC drachm. The significance and origin of this design remains unknown, but it will continue for centuries and would be later incorporated into Roman coinage until at least mid-3rd century AD, making it perhaps one of the longest run ancient numismatic designs. 

The design of inverted heads is unique and only appears in coinage from Istrus of 4th and 3rd centuries BC and could have represented two branches of the Danube, but according to another theory the heads belong to Apollo (the Sun-God), and their inversion was inspired by two nearly total solar eclipses within 3 years from each other visible over Istrus: first in 434 BC and then in 431 BC, during Peloponnesian War. On average at a given location, such an event occurs perhaps once per century; two instances of nearly total solar eclipse in three years was truly extraordinary and may have inspired the design of coins from this region. In any case, there are no similar designs in all of numismatic iconography. Whatever the inspiration for this design was, it was probably a powerful one.