Nero was an infamous brutal dictator that was so bad that his own senate declared him an enemy of the state and decided to have him executed, which he avoided by committing suicide. So in Revelation, John could be saying, “Nero is the second beast,” when he says the number of the beast is 666 (Nero in Gematria).
When you look at the way John allegorically describes this second beast, you can see other possible ways he had Nero in mind. For example, the second beast is said to have a mortal wound that it was healed from. Though it was common knowledge that Nero had committed suicide, there were some legends that he hadn’t really done so and that he would return. Other legends seemed to say that his evil genius would live on and return to rule. It was like he had a mortal wound, but was healed to continue on. While we don’t believe in reincarnation in Christianity, we see the general principle of evil (like Nero) continuing to manifest itself. Many leaders go mad with power and the same kind of characteristics and qualities of Nero resurface in them, painting them in the image of the second beast. Hitler, for example, would be a more modern example of the second beast. His name may not be 666 in Gematria, but he is the antichrist spirit lived on in governmental authority. He is a part of the cycle of what Satan can do when humans give their power over to worship him.
So if the first beast of Revelation is Satan himself (Revelation 13), then the second beast is Satan lived out in humanity, especially when it manifests in government. We often think of state and religion being separate today, but in the ancient world, state was religion. Some leaders claimed to be gods while others claimed to be the mouthpieces of the gods or chosen by the gods. So in Revelation (and the rest of the Bible), spiritual warfare and earthly government is often pictured as intermingling while Christians live as citizens of Heaven under King Jesus. And as the earthly/spiritual intermingling continues in earthly government, it tends to eventually give way to antichrist thinking and methods. Some scholars call it the “domination system”—that is, domination gives rise to domination which gives rise to domination, etc. It’s the constant reestablishment of evil. That being said, so long as we practice the domination ways of the beast (and that goes for America just as much as Germany or Rome), the world will always eventually implode with antichrist.
So antichrist is a thing that has happened (Nero/666), antichrist is a thing that continues to happen (Hitler/modern countries practicing the domination system), and antichrist is a thing that will one day come in fullness (2 Thessalonians 2). And when that fullness of antichrist comes, then Jesus will come to end it. It seems God is just far too patient and loving to give up on humanity until there’s no hope of goodness left in our hearts (2 Peter 3:9).