The Pokemon mascot makes a return in Pokemon Sun and Moon. If you want to catch it, you will have to search around and find it. Check out this guide to figure out How To Get Pikachu In Pokemon Sun And Moon!
Where To Find Pikachu In Pokemon Sun And Moon
Edit – I can confirm that my Pichu evolved into Pikachu at level 22 with max happiness. This is another way of getting Pikachu.
I thought that you could evolve Pichu into Pikachu by becoming friendly with it, that is not the case. You can still catch a Pichu super early in the Hou’Oli City near the Pokemon Lab in any of the grass. If it can evovle into Pikachu, I haven’t figured it out yet. I got mine to level 20 and raised it to max friendship so I’m not sure it can. There is a way to get a Pikachu early still, but you need to progress a bit first. You will need to complete your first trial before this method will work.
I highly recommend something that put Pikachu to sleep or paralyze him. After you beat the first Trial, Pokemon can call for help. Now that Pokemon can call for help you need to go back and find a Pichu again. You need to wait for the Pichu to attack you to trigger the Call For Help. Clearly you don’t want to kill it so use Tail whip or something like that until it attacks. The trigger can take upwards of 10 turns so be ready for the long haul. Then you have to be lucky enough for Pikachu to be the one who comes in to help. When he finally spawns, put him to sleep to make him easier to catch.
You can also try to breed two Pichus and hope that they hatch a Pikachu, but that is still a gamble. I haven’t tried it so I have no idea if it would work or not. It’s not easy but if I have a Pikachu pop up somewhere else with more luck I will update and add that location.