their power is in their mouth and in their tails
The nearest antecedent is the horses (Rev. Rev. 9:17+b) to which the MT and NU texts attest by including the phrase of the horses (τῶν ἵππων [tōn hippōn] ).
their tails are like serpents, having heads; and with them they do harm
The demonic locusts had tails with which they stung (Rev. Rev. 9:10+), but these demonic horses differ in that their tails are like serpents rather than scorpionshaving heads and a mouth. The fierceness of this army can be seen in the lethal power which attends both ends of the horses. Approaching the horsemen from the rear is of little avail since their tails also harm. Some have suggested John is describing modern military weaponry, but the description and the source of the army (demonic, from the pit) is evidence to the contrary.