For their power is in their mouth - That is, as described in the fire, smoke, and brimstone that proceeded out of their mouths. What struck the seer as remarkable on looking on the symbol was, that this immense destruction seemed to proceed out of their mouths. It was not that they trampled down their enemies; nor that they destroyed them with the sword, the bow, or the spear: it was some new and remarkable power in warfare - in which the destruction seemed to proceed from fire, and smoke, and sulphur issuing from the mouths of the horses themselves.
And in their tails - The tails of the horses. This, of course, was something unusual and remarkable in horses, for naturally they have no power there. The power of a fish, or a scorpion, or a wasp, may be said to be in their tails, for their strength or their means of defense or of injury are there; but we never think of speaking in this way of horses. It is not necessary, in the interpretation of this, to suppose that the reference is literally to the tails of the horses, anymore than it is to suppose that the smoke, and fire, and brimstone literally proceeded from their mouths. John describes things as they appeared to him in looking at them from a considerable distance. From their mouths the horses belched forth fire, and smoke, and sulphur, and even their tails seemed to be armed for the work of death.
For their tails were like unto serpents - Not like the tails of serpents, but like serpents themselves.
And had heads - That is, there was something remarkable in the position and appearance of their heads. All serpents, of course, have heads; but John saw something unusual in this - or something so unique in their heads as to attract special attention. It would seem most probable that the heads of these serpents appeared to extend in every direction - as if the hairs of the horses’ tails had been converted into snakes, presenting a most fearful and destructive image. Perhaps it may illustrate this to suppose that there is reference to the Amphisbaena, or two-headed snake. It is said of this reptile that its tail resembles a head, and that with this it throws out its poison (Lucan, vol. ix. p. 179; Pliny’s Hist. Nat. vol. viii. p. 35). It really has but one head, but its tail has the appearance of a head, and it has the power of moving in either direction to a limited degree. If we suppose these snakes fastened to the tail of a horse, the appearance of heads would be very prominent and remarkable. The image is that of the power of destruction. They seemed like ugly and poisonous serpents instead of tails.
And with them they do hurt - Not the main injury, but they have the power of inflicting some injury by them.