"First, return them to their original form, humans, but because of their fondness for horses, let their dicks retain the resemblance of a horses dick a bit.
Secondly, make it so that whenever they get aroused, their dicks swell up to a horses dick again, along with their balls, something the size of a small pony or a Shetland one.
Thirdly, whenever they engage in sexual intercourse let them change into full horses or anthros again, the size of their liking and let their partner adjust to the size, gender and species too.
For the rest add in some quirks of your liking, Beldazzar."