#articuno
There are a lot of legendary Pokémon you’ll encounter in your travels through the Kanto and Johto regions, and Team Neo-Rocket wants ALL of them.
(Want to learn more? Check out the Memories of Kanto tag!)
The first legendary Pokémon you’ll encounter after leaving Lavender Town is Mew, who will appear before you at Mt. Moon, on your way from Cerulean City to Pewter City. This is also the first time you’ll run into Dante, the leader of Team Neo-Rocket, who’s trying to capture Mew with little luck. After this encounter, Mew will become a roaming Pokémon.
It will become steadily more likely to find Mew in tall grass each time you receive a gym badge, and you will gain the ability to track roaming Pokémon via your Pokédex after receiving your fourth badge. The likelihood of encountering Mew will top out at a 50% chance upon stepping into tall grass in the same area as Mew, after you have defeated the final gym in Johto. Mew maintains a consistent level of 50, which will also make it exceedingly difficult to catch, even if you do encounter it early in your journey, to say nothing of the trouble you might have training it without enough gym badges.
The next legendary Pokémon you’ll encounter are the legendary birds.
Articuno and Zapdos have maintained their posts in the Seafoam Islands and the power plant, respectively. Moltres, however, is now found on Mt. Moon. The unusual qualities that attracts Pokémon like Clefairy appears to also draw legendary Pokémon! When you encounter each legendary bird you will also face Neo-Rocket grunts, aiming to tame these powerful, elemental birds. Once you have thwarted Team Neo-Rocket’s exploitative plans you’ll have the opportunity to battle and catch the birds yourself.
However, Team Neo-Rocket has more than just the legendary birds on their agenda. You will encounter them again after you have defeated Erika (the penultimate trainer on your Indigo League gym challenge), when they cause problems on the newly-settled Cinnabar Island.
Team Neo-Rocket are on the island searching for the legendary Pokémon Mewtwo, who they have reason to believe is hidden away in the island’s now-dormant volcano. To this end they have defeated every trainer in the Cinnabar gym, including Fina’s strongest team, and used explosives to damage the gym building. Behind the gym–an area usually blocked off and inaccessible to anyone but gym/lab personnel–is a cave entrance in the side of the volcano.
In enlisting your help to defeat Team Neo-Rocket, Fina explains the situation to you: Mewtwo was created from Mew, and mistreated by the scientists that cloned it from Mew/mutated its DNA. This project was partly funded by Team Rocket, and the goal was to create the absolute most powerful Pokémon. They arguably succeeded, but in doing so they created an angry, cold-hearted being, filled with rage for the control and abuse it suffered at the hands of humans.
In the decades since Mewtwo’s creation, however, it has become more comfortable with humans. Champion Red and Professor Orchid have both worked with Mewtwo to communicate that humans are not exclusively exploitative and evil, and that Pokémon training is a cooperation between trainer and Pokémon, not the absolute control of a Pokémon by its trainer. Leader Fina herself has been working closely with Mewtwo, after her mentor, Blaine, made amends with the legendary Pokémon.
Mewtwo now lives on Cinnabar Island, taking refuge in the caverns and caves of the dormant volcano, which is further maintained through Mewtwo’s power, assuring that the humans and Pokémon on the island will be protected from the natural dangers of the volcano. But now, Team Neo-Rocket are barging into Mewtwo’s refuge, with the full intent of destroying it, capturing the Pokémon that have found protection here and forcing Mewtwo back into servitude for the benefit of Team Neo-Rocket.
In the caves and the heart of the volcano, you’ll find many lush, grassy spaces amongst the rocks, and a wide variety of Pokémon. Most notably, this island hideaway is home to Growlithes/Archenines, Dratinis/Dragonairs, Magbys/Magmars, and is one of the few places where you can encounter Crobat in the wild.
Throughout the caves, Neo-Rocket grunts will work to stop you in your tracks, and when you finally reach the deepest cavern in the heart of the volcano, the leader of Team Neo-Rocket, Dante, challenging Mewtwo. You’ll need to battle Dante to protect Mewtwo’s new home, and all of the Pokémon who live here!
Once you have defeated Dante, Mewtwo will comment that they were impressed by your skill in battle and your closeness with your Pokémon. Because of this, Mewtwo makes an offer to you that, once you have obtained your final gym badge, if you can defeat this legendary Pokémon in battle, Mewtwo will join your party. As such, you can return to Cinnabar island any time after defeating gym leader Blue, and take on Mewtwo.
Their defeat on Cinnabar Island will have weakened Team Neo-Rocket, so your next step will be to take them on in their own headquarters, back in Vermillion City. Once defeated on their own turf, Team Neo-Rocket will retreat deeper into the shadows.
However, they aren’t truly defeated yet. Upon entering the Kanto hall of fame, you’ll discover that Team Neo-Rocket has reappeared in the neighboring region of Johto, and they are still dead-set on securing power through legendary Pokémon…