Zapdon

The legendaries your able to catch are the following:
Zapdos:Lv50:PowerPlant:Fly to the Pokemon center around that Cave above Lavender(fly not to lavender) then surf down. If you're playing FireRed you should be able to get a Elelktebuz.
Articuno:Lv50:SeafoomIslands:There are 2 ways to getting to this island but the only way to get there for Articuno is by surfing from Fushia. At some point you'll need strength
Moltres:Lv50:Mt.Ember:When you beat the 7th gym and Bill takes you to One Island, this would be the wisest time to get him. Instead of going to Two Island, go north to Mt.Ember. You should get RockSmash in a spa on the way and make sure you have strength.
Mewtwo:Lv70:CeruleanCave:Beat the Elite four, have 60 Pokemon in pokedex, go to Celio and get the Ruby and Sapphire for him. After that Cerulean cave should be accesible. All you need to get to Mewtwo is surf, if you come across a boulder, roll or smash, you are going the wrong way. I'd suggest you get a Masterball for him.
Raikou, Entei, Suicune:Lv.40/50(?):Everywhere in Kanto: This is the same as Latios and Latias in Ruby and Sapphire and these same Pokemon in Gold, Silver and Crystal, except you get only one. You get the one that has a type advantage over your original starter, so it's picked at the beggining.
Lugia/Ho-oh: Lv70(each):Navel Rock:NintendoEvent
Deoxys:Lv30:Birth Island:NintendoEvent

Other pokemon:
Mew:Emerald:FarAwayIsland:NintendoEvent
Latios/Latias:Emerald/Ruby/Sapphire:Everywhere
Groudon:Emerald/Ruby:TerraCave(Emerald) or Cave of Origin(Ruby)
Kyogre:Emerald/Sapphire:MarineCave(Emerald) or Cave of Origin(Sapphire)
Registeel,Regirock and Regice:Emerald/Ruby/Sapphire:Caves around Hoenn(Activate by going to sealed chamber)
Lugia/Ho-oh: Lv70(each):Emerald/Leafgreen/FireRed:Navel Rock:NintendoEvent
Deoxys:Lv30:Emerald/Leafgreen/FireRed:Birth Island:NintendoEvent
Celebi:Lv???:Pokemon10th anneverseryNation Tour(www.pokemon.com)This is the only way to get Celebi.