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.
Here we have the mystery of this vision explained. The apostle wonders at the sight of this woman: the angel undertakes to open this vision to him, it being the key of the former visions; and he tells the apostle what was meant by the beast on which the woman sat; but it is so explained as still to need further explanation. 1.
Commenting on Revelation 17:7-13
And the ten horns which thou sawest are ten kings,.... Not ten Christian emperors, which are reckoned up by Brightman from Constantine to Theodosius; for these did not reign with the beast, or give their kingdoms to him, and much less did they make war with the Lamb; they are rather the angels of Michael, that fought for him, the Lamb, against the dragon, and...
Verse 12. And the ten horns which thou sawest. On the scarlet-coloured beast, . Are ten kings. Represent or denote ten kings; that is, kingdoms or powers. . Which have received no kingdom as yet. That is, they were not in existence when John wrote.