The grazing horse design pops up on coins all over the ancient Greek world (most recognizably on those of Larissa in Thessaly). I'm not sure why it was specifically chosen to be used in this area, but as early as the 3rd century BC, we see them on bronze issues of this city. In Troy the Coins, Bellinger states that the type was simply copied from earlier issues of Neandria, one of the cities subsumed into Antigonus Troas (which later became Alexandria Troas) when Antigonus refounded the city around 306 BC. Here's one of those bronzes, a really small one : TROAS, Alexandria Troas Circa 3rd - 2nd centuries BC AE9 0.62g, 9.2mm BMC 9 var (grape bunch instead of wreath below horse) O: Laureate head of Apollo right. R: ALE above horse grazing right, bunch of grapes beneath. I do like the variations that we see on the Roman Provincial issues, with the tree (and sometimes shepherd) together with the horse. Your two new coins are both very nice, the Gallienus particularly so .