It depends on if you want ho-oh right away and don't have the time to weaken it, you can if you want but i would recommend using it for Lugia if you can catch it in gold. Hope this helps =P
I would recommend that if you are using the Gold version, use it for Lugia (since it would be level 70 while Houou I think is 40 when you catch it), and for Houou in the Silver version (since if then, Houou will be the level 70 Pokemon)
But personally, I would use it on a legendary dogs. They are much harder to catch because they escape you a lot, unless you have a move like Spiderweb or Mean Look equal to or under Lv 40 (Crystal) or 50 (G/S) to use a repel in a place w/o Pokemon in the aforementioned level range.
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No but i have heard that you can use it like a master ball on latias
no, you don't have to use your master ball on it. you could use it for any legendary Pokemon like mewtwo, or... you know what i mean!
master. no other ball can hold it.
Use Kygore in the cave of origins you don't even have to attack! Walk up to him the orb should be glowing and then the battle, use the master ball and done! 1 Kygore! But there's one on master ball in the entire game! So use it here.
you can use master ball in caught the groudon