There are several different positions. The Historicist view seems plausible. The Geneva Bible footnotes say;
p. Which is ment of Julius Cesar, Augustus, Tiberius, Caligula, Claudius, and Nero &c. who were as Kings in effect, but because thei colde not rule, but by the consent of the Senat, their power is compared to a litle horne For Mahomet came not of the Romaine empire, and the Pope hathe no vocacion of governement: therefore this can not be applied unto them and also in this prophecie the Prophets purpose is chiefly to comfort the Jewes unto the revelation of Christ Some take it for the whole bodie of Antichrist.
q. Meaning, a certeine portion of the ten hornes: that is, a parte from the whole estate was taken away. For Augustus toke from the Senat the libertie of chosing the deputes to send into the provinces, and toke the governement of certeine countreis to him self.
r. These Romaine Emperours at the first used a certeine humanitie and gentlenes, and were content that others as the Consuls and Senat shulde beare the name of dignitie, so that thei might have the profite, and therefore in elections and counsels wolde behave them selves according as did other Senatours: yet against their enemies and those that wolde resist them, thei were fierce and cruel, which is here ment by the proud mouth.
Geneva Bible: Notes. (1560). (Vol. 1, p. 361). Geneva: Rovland Hall.