Unlike the “seven kings” of Revelation 17:10, “five of whom have fallen” (in the past), these ten horns are presented as yet to come (“have not yet received royal power”). Here John is told that in time to come (future for John in the first century AD) the beast will utilize its horns as it works alongside the dragon (Revelation 12:17) to make war against the saints (Revelation 13:7). To say it differently, kings use mighty men to bully God’s people into spiritual adultery with the great prostitute.
12 And the ten horns that you saw are ten kings who have not yet received royal power, but they are to receive authority as kings for one hour, together with the beast.