IIRC, the civic coppers were a market tax: the coppers had a limited validity (one year?) and had to be re-minted often. At a fee, of course. This also explains the frequent overstrikes. If there were a zodiac series, they could help people to sort old coins and new ones quickly.
In Islam, it is not allowed to picture living animals, but, as you know, it is often allowed to make a picture of non-existant animals. Therefore, a dragon would be suitable, but other zodiac animals would not be. However, since the civic coppers show many other animals (lion, horse, stag etc.) the city mints seem to have enjoyed a degree of flexibility.
Peter