The book of Revelation begins with the words "That which was, which is and which is to come" so I suppose from that we have past, present and future with John writing in approximately 96 AD being the axis. Thus that which "was" has already past, that which "is" speaks of the time of John and that which "is to come" speaks of all that which proceeds John. As John was the last of the twelve apostles it seems reasonable that he would have been the one chosen of the Lord Himself to make these things available to the church in that authority as one who was with the Lord, beginning at the baptism of John and being witnesses of His death and resurrection etc.
As far as who the beast is John says that he is, was, is not, and is about to come up out of the abyss. So again we can easily see that the beast (who is a man) was no longer alive though he had lived before John writing in 96 AD, that he was not alive in 96 AD and that he will come up out of the abyss at some unspecified date after 96 AD.
Again as for the more biographical information we have of this man (the beast) we read that he is one of seven kings, and will be an eight king as well. So he was a king, being one of seven kings, and he will become an eight king. There are seven kings but one of these kings reigns twice. Finally we are told biographically that the number of his name is 666. Given that John clearly expected the reader at that time to comprehend who he was speaking about and that the term king is derived from the latin "Caesar" it isn't too difficult to accept that John was writing about one of seven men who were Caesars (kings). Nor is it difficult to know which one of these Caesars he was writing about.
Augustus (Imp. Caesar Augustus)\t 27 BC-AD 14
Tiberius (Ti. Caesar Augustus)\t AD 14-37
Gaius / Caligula (C. Caesar Augustus Germanicus)\t 37-41
Claudius (Ti. Claudius Caesar Augustus Germanicus)\t 41-54
Nero (Imp. Nero Claudius Caesar Augustus Germanicus)\t 54-68
Galba (Ser. Sulpicius Galba Imp. Caesar Augustus)\t 68-69
Otho (Imp. M. Otho Caesar Augustus)\t 69
Vitellius (A. Vitellius Augustus Germanicus Imp.)\t 69
Vespasian (Imp. Caesar Vespasianus Augustus)\t 69-79
Titus (Imp. Titus Caesar Vespasianus Augustus)\t 79-81
Domitian (Imp. Caesar Domitianus Augustus)\t 81-96
Of the twelve Caesars who reigned up to and including 96 AD, Julius Caesar was the first, and in 44 BC, the year he was assassinated, he declared himself god. His adopted son Octavian didn't become Caesar until he had fought a number of civil wars and so there was a gap of 17 years between the death of Julius Caesar and the declaration of Augustus Caesar (Octavian). Thereafter the Caesars reigned until the end of the first century, technically ending with Domitian as the last (Prophetic) Caesar and the one who sentenced John to the labour mine on Patmos where he wrote the book of Revelation in his last year of imprisonment. Of the twelve (Prophetic) line of Caesars seven spoke as god and five did not speak as god. The sixth (measured from Julius) or the fifth (measured from Augustus) was Nero and he is the last of the Julio-Claudian line. After Nero there were four men who became Caesar in a single 12 month period, the last of which was Vespasian. who was Caesar for ten years and was replaced by Domitian was the last of the (prophetic) caesars.
Therefore when John was writing from his imprisonment on the Island of Patmos he was writing in the reign of Domitian who had banished him there. Thus when it is written:
""The beast that you saw was, and is not, and is about to come up out of the abyss and go to destruction. And those who dwell on the earth, whose name has not been written in the book of life from the foundation of the world, will wonder when they see the beast, that he was and is not and will come. "Here is the mind which has wisdom. The seven heads are seven mountains on which the woman sits, and they are seven kings; five have fallen, one is, the other has not yet come; and when he comes, he must remain a little while. "The beast which was and is not, is himself also an eighth and is one of the seven, and he goes to destruction. "The ten horns which you saw are ten kings who have not yet received a kingdom, but they receive authority as kings with the beast for one hour. "These have one purpose, and they give their power and authority to the beast.” (Revelation 17:8”“13, NASB95)
.....we are told plainly (at the time John was writing) that of these seven kings, five have fallen, one is, and the other has not yet come. I will leave you to work out who he is then from this information. The key is that he is a prophetic king because he speaks as god. It is clear and cannot be shaken. The key to understanding who the beast of Revelation 17:8 is as a future event from the time of John (after the destruction of Jerusalem of course) is that not only is he one of the seven, the seventh of which had not yet come, but he was the fifth of these seven and also an eighth.
Just to add a curious fact, this prophecy which John says attends to wisdom also mentions ten kings. At the moment there are nine reigning Monarchs in Europe in the dominion of the old Roman Empire. The tenth and the oldest Monarch in Europe is of course The Pope.
From The Vatican State Official Website:
Vatican City State is governed as an absolute monarchy. The Head of State is the Pope who holds full legislative, executive and judicial powers.
On the basis of this the Pope cannot be the beast of revelations 17:8. Still he is a curious fella!