12 “The ten horns you saw are ten kings who have not yet received a kingdom, but who for one hour will receive authority as kings along with the beast.

Read Revelation 17:12 Using Other Translations

And the ten horns which thou sawest are ten kings, which have received no kingdom as yet; but receive power as kings one hour with the beast.
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.
The ten horns of the beast are ten kings who have not yet risen to power. They will be appointed to their kingdoms for one brief moment to reign with the beast.