The "one who is" indicated the present tense when the book of Revelation was written. Christ was telling John of the destruction that was about to come upon Jerusalem in the 1st century AD. So, the ruling beast king had to be the sixth king of the 4th beast kingdom that Daniel ch. 2, 7, & 9 had prophesied would be in power when Christ was "cut off" - crucified.
Those markers limit that 4th beast kingdom to the ancient Roman empire of the 1st century AD. Therefor, the sixth ruler of the ancient Roman empire was the "beast" ruler or king that was ruling that sea beast nation when the book was written. We count them:
- Julius Caesar, 2) Octavian Augustus, 3) Tiberius, 4) Caligula, 5) Claudius, and 6) Nero.
Nero was the "beast" ruler at the time Revelation was written. So we can know that Revelation was written before the temple in Jerusalem was destroyed.
The seventh ruler was Galba who assumed power at Nero's death and ruled for about 6 months. The 8th king then was Otho, and the 9th was Vitelius. Then Vespasian took control. The year of four emperors from 68 - 69 AD saw civil wars that nearly destroyed Rome. It was Vespasian who pulled it back from the brink. (1)
Each ruling king of the beast nation took on the role of a beast king. Each head of the beast was also referred to as a beast. So the beast kingdom of the sea and the ruling king of it were interchangeably referred to as "the beast". When the sixth beast (Nero) was mortally wounded unto death, the entire beast kingdom almost fell.
Beasts were / are oppressor nations / kingdoms that preyed upon the people. The word beast in Rev. 13:1 is “therion” and means a wild beast, a predator that preys upon and devours the young and the weak.(2) God used this symbol of a wild beast (lions, bears, leopards, wolves, etc.) for oppressive, parasitic kingdoms that metaphorically fed off the people, draining their wealth through wars, taxation, and theft.
Nero “was and is not”, meaning he had died (Rev. 17:11). He was the last of the Julian line of Caesars. But, he is shown again in the eighth beast ruler. The eighth Caesar of Rome was Otho.
Otho murdered Galba, and was proclaimed emperor by the army. He then ruled for three months, from Jan to April AD 69. He had been a friend to Nero, but had joined with Galba to overthrow Nero. (3), (4), (5) That Otho was “of the seven” possibly refers to his conspiracy with Galba to overthrow Nero.
Vitellius’ army defeated that of Otho in April of AD 69. He ruled Rome for approx. 7 months until Dec AD 69. He had also been a companion to Nero, and signaled the type of rule he would follow by making funerary offerings to Nero and calling for Nero’s songs. (6). He was a very cruel and gluttonous man, and soon lost support of the armies. His army was defeated by Vespasian’s, and the senate confirmed Vespasian as emperor on Dec. 21, AD 69. (7) (8)
Tacitus wrote, “Moreover, on certain days the people and soldiers, as if adding thereby to Otho’s nobility and distinction, acclaimed him as Nero Otho.” (Histories 1.78; cf. 1.13 and 1.25). Note:19 ” On the portrayal of Otho in Tacitus, Plutarch and Suetonius, see Perkins 1993 and Braun 1992.”
Nero, one head of the 4th beast empire, was not the entire beastly empire. But, two of his successors, Otho and Vitellius both honored and ruled in Nero’s memory, in essence making a reappearance of Nero and “ascending out of the bottomless pit” (Rev. 17:8) Both went "into perdition" very quickly.
See more on Rev. 13 at my blog ShreddingTheVeil.org:
Frequent Mistakes - Part 1: Rev. 13:3 The Wounded Head here
The Twelve Caesars - here
Therion - Strong's Gr 2342 Biblehub
“Otho” from Encyclopedia Britannica here
Otho from Ancient History Encyclopedia here
Marcus Salvius Otho from Roman-Empire.net here
The Life of Vitellius by Seutonius here
Vitellius from Ancient History here
Titus Flavius Sabinus Vespasianus from Roman-Empire.net here