19:18in order that you may eat the flesh of kings and the flesh of commanders and the flesh of mighty men and the flesh of horses and of those who sit on them and the flesh of all men, both free men and slaves, and small and great."

in order that you may eat

There is no need to delineate the meaning of each of the persons involved. Rather 'THE FLESH OF' has been highlighted to show that this is more of a spiritual meal (previous verse). When Adam and Eve followed the advice of Satan and ate fruit from the tree of knowledge of GOOD and EVIL (Genesis 3:1-7), sin entered into this world. By the time of this verse in Revelation, the GOOD people are already in heaven and probably are on those white horses coming with Christ (verse 14), so these are the ones who followed EVIL due to the deception of the beast and the false prophet (verse 20). So here we see EVIL being removed from ALL MEN (left behind).

Including horses doesn't seem to fit. Probably these are the EVIL counterpart to what believers ride upon (verse 14). Also, we had locusts (Revelation 9:7) that looked like horses prepared for battle, and these were EVIL influences. Horses may denote how EVIL is transported. We don't understand this phenomenon, i.e. how does EVIL enter a man (Luke 22:1-3-6)?

Lastly, lest we think this is all spiritual, the literal consumption of ALL MEN remaining by these birds will take place. This leaves the interesting question: who will populate the earth during the millennial reign (Revelation 20:4-6)?