John Gill Commentary Revelation 9:19

"For the power of the horses is in their mouth, and in their tails: for their tails are like unto serpents, and have heads; and with them they hurt." — Revelation 9:19 (ASV)

For their power is in their mouth
In what proceeded out of their mouth, or seethed to do so; in their guns, and what came out of them:

and in their tails ;
which may design their foot soldiers, which were as the tail to their horse, and who sometimes did great service; or their way of fighting when they fled, by casting up arrows into the air, which would fall upon the heads and horses of those that pursued them; or their ambushments, by which they destroyed many; or their perfidious violation of treaties; or it may be their tails may intend the doctrine of Mahomet, the false prophet, who is the tail, (Isaiah 9:15) ,

for their tails [were] like unto serpents ;
crooked, crafty, poisonous, and, pernicious:

and had heads ;
every tail had a head to it; which may be understood of the officers of the foot soldiers, or of the priests and teachers of the Mahometan religion:

and with them they do hurt ;
with their guns, the power in their mouth, they did hurt to the bodies of men; and with their false doctrines, their tails, they did hurt to the souls of men; the Ethiopic version here adds, "five months"; which seems to be taken from (Revelation 9:10) .