The final scene of the latest episode of Arrow, "Present Tense," may have just introduced the Anti-Monitor ahead of Crisis on Infinite Earths. The plot of Arrow season 8 has centered upon Oliver Queen, as he travels across time and space to aid the cosmic being known as the Monitor in preparing for an approaching crisis that threatens the entire multiverse. Oliver agreed to do this as part of a bargain he struck with the Monitor during the Elseworlds crossover event, in order to save the lives of The Flash and Supergirl. Recently, Oliver came to question the Monitor's true motivations and began seeking out information about him, discovering that he (or a cosmic being very much like him) was responsible for establishing the League of Assassins over ten centuries earlier.
Oliver had good reason to be suspicious, given what activities viewers had seen the Monitor involved in elsewhere in the Arrowverse. It appears that the Monitor may have resurrected Lex Luthor on Earth-38 and he certainly arranged for Martian Manhunter's twisted brother Malefic to be released from the Phantom Zone. The fact that the Monitor's original plan for finding worthy heroes to help him save reality from the coming Crisis involved testing heroes by giving lunatics the power to alter reality and letting whole universes die if they failed the test (as in the opening scene of Elseworlds) also cannot be ignored. Despite this, there is still reason to doubt that the Monitor in "Present Tense" is the same Monitor we've seen elsewhere around the Arrowverse, and is in fact the Anti-Monitor.
The closing scene of Arrow, season 8, episode 4, "Present Tense," saw the Monitor appear to the Laurel Lance of Earth-2, as she was leaving Star City Police Headquarters after a meeting with Dinah Drake. She guessed his identity immediately, which amused the Monitor, who asked how she knew he was the Monitor. Her sarcastic response was that "no human would actually wear something so ridiculous." Undeterred, the Monitor said that he knew she was hurting because of the destruction of Earth-2 and that it was in his power to restore her universe. All the Monitor required was that she accomplish a single task: betray Oliver Queen at a key moment in the future.
On the surface, it makes no sense for the Monitor to be making this kind of deal, pitting another hero against his most active agent, even if he fears Oliver turning against him due to his investigation into his past activities on Earth-1. It is also notable that the Monitor's offer to Laurel is far more straight-forward than any of his past interactions with mere mortals, with the Monitor being fond of speaking in allegory and flowery, poetic language. Indeed, the whole reason Oliver began researching the Monitor's past was because of his inability to get a straight answer about anything out of the cosmic being.
All of this coupled with the fact that the Monitor at the end of "Present Tense" outright asks Laurel how she can be certain he is the Monitor suggests that he is, in fact, a disguised Anti-Monitor. While the Monitor has a history of sinister behavior and working with less than respectable characters, it makes much more sense for the Anti-Monitor to seek out Laurel's assistance and trick her into helping his cause ahead of Crisis on Infinite Earths. The big question regarding the rest of Arrow season 8 is if the former Black Siren will remain true to her new allies or if she will betray them all once again for the sake of her world.