If we refer to Revelation 17, then Babylon is a metaphor of the Roman Empire.
The "seven mountain" refers to the
Seven Hills that formed the geographical heart of Rome. Vespasian minted several coins during his office with one of them depicting a woman sitting on seven hills on the reverse side of the coin with the word "Roma" written underneath the image.
Suetonius, a Roman historian, excludes the short reign of Galba, Otho, and Vitellius as rebels of the empire, and may not have included them as legitimate rulers. If this is true, and we were to work the seven kings (Revelation 17:10) from Augustus, then the "one that is" refers to Vespasian, and the "other" yet to come would be his son Titus, and the eighth "which belongs to the seven," which personifies the beast, would be Domitian, Vespasian's other son. This fits with the fact that Vespasian and Titus were responsible for the destruction of the Second Temple under Nero's office.
However, I am taking a sort of preterist view of these particular portions of the book, so my interpretation could be wrong.