Hi I'm an archaeology major and it actually depends on the era! If they are drawn on vases to be men with the back end of horses they have dicks like men. There is then a shift to bottom half of horses in later Greek vase painting and then they have dicks like horses.
I suddenly remembered a meme about how centaurs have sex. Is their dick in front like a man or at the under back part like a horse looool #-.-)