The beast that was and is not … is an eighth: this means that the beast that will come back from the abyss (verse 8) will become the eighth king. As stated in verse 8, he is doomed to destruction.

It belongs to the seven: the Greek seems to mean, as Good News Translation and others translate it, that “it is one of the seven.” That is, the eighth king will be one of the seven previous kings; the text does not say which one of them he will be. This explains how the seven heads can represent eight kings; one of the kings will rule twice.

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on The Revelation to John. (UBS Handbook Series). New York: UBS, 1993. For this and other handbooks for translators see here .