Like Sun and Moon, Pokemon Sword and Shield introduce new forms of old Pokemon to fit the games' region. These Galarian forms are loosely British-inspired--there's a Charlie Chaplin-like evolution for Mr. Mime called Mr. Rime, for instance. On the more Brit punk side of things, there's Galarian Zigzagoon and Galarian Linoone, black-and-white versions of the more run-of-the-mill ones first introduced in Ruby and Sapphire. And with their Galar forms, they also get a third evolution: the Gene Simmons-looking Obstagoon.
Galarian Zigzagoon evolves into Linoone by leveling up as normal. To get Obstagoon, though, you have to put your Linoone to work. Linoone only evolves if it's participated in lots of battles--while I don't have an exact number, mine was in my team from nearly the beginning of the game, and it didn't evolve until around level 55 or so. The Pokedex indicates that the Galarian forms of these Pokemon came about because they have harsh street-urchin lives, so it makes sense that getting into lots of scrapes would turn one into a monster that looks like it's in Kiss.
You can also catch Obstagoon in the wild once you've collected enough badges. Depending on the weather conditions, it'll show up in the Bridge Field region of the Wild Area, sort of near the Daycare. You can also find one past the lake next to Professor Magnolia's house if you ride your bike all the way west (it's technically Route 2). You'll want at least one Obstagoon you're willing to part with, because you can trade one to a Team Yell Grunt in Spikemuth for a Kantonian Mr. Mime.
Obstagoon learns an interesting version of Protect called Obstruct. Direct contact from an opponent will harshly lower that opponent's Defense stat, which helps make Obstagoon's attacks hit harder while also protecting it for a turn.
For more on Sword and Shield, be sure to read our review. There are other Gen 8 Pokemon that are tricky to evolve, but we have guides on them, too! See how to evolve Milcery into Alremie, Sinistea into Polteageist, and Applin into either Flapple or Appletun.