The beast existed earlier. He was not around when John wrote Revelation, but he will appear in the for a short time and then go to destruction. Some people believe that Nero will be reincarnated in the form of the Antichrist. Some think that it will be Antiochus IV Epiphanes. Either way, they think some really bad guy will come back or the Antichrist will have the same spirit as him..
8 All inhabitants of the earth will worship the beast—all whose names have not been written in the Lamb’s book of life, the Lamb who was slain from the creation of the world.
καὶ προσκυνήσουσιν αὐτὸν πάντες οἱ κατοικοῦντες ἐπὶ τῆς γῆς οὗ οὐ γέγραπται τὸ ὄνομα αὐτοῦ ἐν τῷ βιβλίῳ τῆς ζωῆς τοῦ ἀρνίου τοῦ ἐσφαγμένου ἀπὸ καταβολῆς κόσμου
NOTE: This verse makes it clear that it is the Lamb who was slain from the foundation of the world (NOT the names written from the foundation of the world).
It is also possible to interpret this as being a book that has existed from the foundation of the world.
Some people believe that this verse teaches that some people's names are written in the book and are predestined to be saved. But this interpretation goes against the biblical texts that describe the book of life as having names that are written and later blotted out if the person sins and is unrepentant. It also ignores Rev. 13;8 in which it is the Lamb who has been slain from the foundation of the world (Not names written from the foundation of the world) (See comments for Rev. 13:8).