John had seen on the seven heads of the woman’s mount ten horns (Revelation 17:3, Revelation 17:7). Horns are the means that goats (for example) use to insist on their own way (Ezekiel 34:21). So the “horns” represent bullies, men of might who use their power (be it wealth, position, etc.) to push their own agenda. The number “ten” is again symbolic, denoting a fullness of bullies, all the bullies of (future) history. Horns are on heads, and so we are to understand that these ten bullies are not separate from the seven kings but connected to them; they use their horns to get their way.
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.