so... you play POKEMON SILVER... okay, here goes... if you play silver, you will get the rainbow wing from the old man in front of viridian city gym (if you got the s.s. ticket). then, go back to johto and go up the tin tower. battle HO-OH at the top (that's the reason why you need the rainbow feather: to catch HO-OH!) and if you manage to catch it, HO-OH's yours!