A.T. Robertson 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)

The power (η εξουσια). As in 2:26; 6:8. This power of the horses is both in their mouths (because of the fire, smoke, brimstone) and in their tails, "for their tails are like unto serpents" (α γαρ ουρα αυτων ομοια οφεσιν). Associative-instrumental case οφεσιν after ομοια. Οφις is old word for snake (Matthew 7:10).

Having heads (εχουσα κεφαλας). Feminine present active participle of εχω, agreeing with ουρα (tails).

With them (εν αυταις). Instrumental use of εν. Surely dreadful monsters.