If you mean the rainbow wing you have to beat Team Rocket at Goldenrod Radio Tower. After that the director will come and give you the Rainbow Wing.
P.S But to get Ho-oh you have to have all the gym badges and beat the Kimono Girls in the Dance Theater then they will give you the Clear Bell
in goldenrod city
Save it for later
you show him the rainbow wing
Rainbow Wing and Clear Bell
You first have to beat team rocket, then the Kimono girls
Once you get to pewter city a man somewhere in the city outside will give you the rainbow wing (how easy).
you get the rainbow wing from the president in the radio tower along the lines of the story
Get the rainbow wing from the radio tower's director and then go to pro.Elm.
go t the radio tower and save the manager
the name of the Pokemon on the cover of heart gold (the one that looks like a bird with a rainbow on it) is HO-OH
After you beat Archer (The neo team Rocket Leader) the directer gives it to you
After you beat the last team rocket executive on the observation deck, in the radio tower in goldenrod city, you get the rainbow wing from the Rainbow wing from the director.