The “Surprised Pikachu” meme is a screen capture of the Pokemon character Pikachu with his mouth wide open, usually accompanied by text. When unaccompanied by text, it can be used as a (mocking) reaction image. Paired with text, it becomes an even bigger mockery of an obvious outcome that should have been but wasn't predicted. So far, it's the most-used meme of 2018, and one of the biggest ones of the past few years.
This viral sensation got its start on Tumblr, on 26 September 2018. This post is the first known example of it being used, but that's not where it gained its major popularity. It was reposted on Reddit's viral image boards later that week, where it reached a wide audience and inspired many more variations. From there, it spread to Facebook, Twitter, 4chan, and many other social networking websites.
Season 1, Episode 10, "Bulbasaur and the Hidden Village" (3:05), 3 June 1997 is the episode from the Pokémon anime where the image comes from.
'Surprised Pikachu' is primarily used as a sarcastic reaction image. The meaning behind using it is to convey a sarcastic sense of shock at an outcome that was fairly obvious and predictable. The first use of it perfectly conveys the meaning and use of the image at large.
Due to its adaptable nature and the meaning conveyed by the image, 'Surprised Pikachu' is used by many people and many fandoms. The popularity of it has soared dramatically, as well. In just a few weeks it was able to become the most-used viral image on many subReddits as well as the iFunny app. With how close 2018 is to ending, 'Surprised Pikachu' will probably remain the #1 meme of the year.