You’ll get the Rainbow Wing from a monk in Tin Tower in Ecruteak City. But you’ll first need to beat the Pokémon League and catch all three Legendary Beasts (Suicune, Raikou, and Entei), so you’ll need to be into the post-game before this item is available.
After you receive the Rainbow Wing, a set of stairs will appear beside you in Tin Tower. Use them to begin the difficult climb to meet the Legendary Pokémon Ho-Oh.
Step 1: Beating the Elite IV and Champion Lance
The Elite IV and Champion Lance can be fought at the Indigo Plateau after collecting all eight Johto Gym Badges.
You’ll have to use Surf to go east of New Bark town into Route 27. Then Waterfall is needed to cross the Tohjo Falls.
Continuing in this direction will take you through Routes 27 and 26, the Pokémon League Reception Gate, and finally the Victory Road before reaching the Indigo Plateau.
Once you beat the Elite Four and Champion, you’ll officially be in the post-game and will be able to chase after the three legendary beasts.
Step 2: Catching the Legendary Beasts
Finding and catching the Legendary Beasts is no easy task, but it’s necessary if you want the Rainbow Wing.
Suicune is the easiest since you’ll get a chance to battle and catch it as part of the main storyline. It happens after you get the Clear Bell and enter the Tin Tower for the first time.
Tip: Make sure you catch Suicune instead of making it faint. You won’t get any more chances to catch that Pokémon, so making it faint would lock you out of catching Ho-Oh.
Raikou and Entei are another story.
Widely considered to be the trickiest catches in Pokémon Crystal, these two freely roam Johto after you first meet them in the Tin Tower.
Bumping into them in the wild is a matter of chance, and they’ll flee the moment you do.
A Pokémon with Mean Look can stop them from fleeing, but that only makes things a little easier.
Step 3: Getting the Rainbow Wing
Once you’ve entered the Hall of Fame and caught all three Legendary Beasts, make your way to the Tin Tower in Ecruteak City.
Tip: You don’t need to have the beasts in your party for this. As long as they’re all in your PC, you can get the Rainbow Wing.
Enter the building next to the pond in the northern part of town and follow the only available road.
After passing through some stairs and hallways, you’ll reach the Tin Tower. Go inside.
Talk to the monk in the back of the first floor.
He’ll give you the Rainbow Wing as a means of protection, which will open the way forward.
Using the Rainbow Wing to Reach Ho-Oh
After getting the Rainbow Wing, a new set of stairs will appear beside you.
Going up these stairs is the first step in the long journey toward Ho-Oh.
Floors 2-9 are one giant maze full of puzzles and traps that can get frustrating – but keep going and you’ll finally reach the roof (10F).
Ho-Oh awaits you there.
Tip: Make sure to save before interacting with the sacred bird.