The incuse divided square places us among the Greek coinages of the Archaic period. One of my references shows a tetradrachm of Potidaia with such a reverse, the obverse being a man on horseback carrying a trident, which is perhaps suggested by the diagonal lines here, but that coin has a large star in the lower part of the field below the horse. There is also an octodrachm of Bisaltae with an incuse-square reverse, and the figure of a man standing beside a horse, with some kind of decoration or lettering in the field.
It looks rather less like the horseback rider of Erythrae, and otherwise the closest resemblance I can find in my (admittedly limited) research materials is to the motif of Silenus standing beside his ass on some coins of Mende. Hopefully someone else can give better advice.