There is a new Pokemon released in Pokemon Go named Smeargle, how do you catch him? He's not available in the wild.
While TurkuSama's answer pretty much sums up how to find Smeargle, there are a few other things are worth pointing out
By using the new photo feature, Smeargle has a random chance to photobomb the picture you have taken. While it is random, it was been confirmed by Niantic Support that Smeargle will only appear in the first photo in session, and you can increase your chances by taking pictures of a variety of Pokemon
[...] Smeargle appears only in the first photo of a session. To increase your chances to encounter one, try taking pictures with more variety of Pokemon
Smeargle can be found in both AR and AR+ picture modes. Standard AR mode is much faster when Smeargle hunting. AR+ requires you to look around and place your Pokemon in the area. This can take a lot of time
A few reddit posts are reporting that Smeargle will only photobomb once per day. After you find a Smeargle, you can find another one after midnight.
An easy way to tell if Smeargle photobomed your picture, without having to backout and review the photo, is to monitor the mini view icon on the bottom right of your screen. When a photo is taken, it will burst a single white circle. If Smeargle is there, the icon will shimmer and pulse three times.
Once Smeargle has photobombed your photo, return to the world map. Smeargle will spawn in the wild.
Smeargle will stay in the area for roughly 1 hour from the time it spawns.
Catching mechanics for Smeargle is the same as a regular spawn. It can run (my first one ran on me)
Smeargle's move set will copy the Pokemon it photobombed, except for Frustration and Return. If the Pokemon contains either of these two moves, Smeargle will instead obtain a random move in replace of these.