Who says it can’t be both? I think I may have the milk taste sweeter and richer than normal, making it high in demand, and more nutritious. Under certain circumstances, perhaps through direct ingestion, or bottled in a vacuumed circumstance (milk machine perhaps) there could be latent magic mixed with the milk that could cause the drinker to become a cowpony.
Of course, a full transition would probably take a lot of consumption, and the effects could even be temporary if not fully realized.
All things are under consideration in this matter.