The ten horns you saw are ten kings who have not yet received a kingdom, but will receive one hour of authority as kings along with the beast.
KJV
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.
ASV
And the ten horns that thou sawest are ten kings, who have received no kingdom as yet; but they receive authority as kings, with the beast, for one hour.