{The power} (h(8820)exousia). 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" (hai gar ourai aut(936e) homoiai ophesin).
Associative-instrumental case ophesin after homoiai. Ophis
is old word for snake ( Mt 7:10 ). {Having heads} (echousai
kephalas). Feminine present active participle of ech(935c),
agreeing with ourai (tails). {With them} (en autais).
Instrumental use of en. Surely dreadful monsters.