You can find Legendary Pokemon as follows:Articuno: go to Seafoam Islands and solve the rock puzzles to get it however it is not available in Diamond or Pearl so you can migrate it over to Diamond or Pearl and while you can migrate to Platinum as well you can find a wild one in Platinum talk to Professor Oak and go to his house and the birds will start roaming around.
Zapdos: Go to the Power Plant by Rock Tunnel and walk through it until you get to the end to find it, you can also migrate to Diamond and Pearl since it is not available there however while it is available in Platinum you can still migrate it into Platinum as well, in order to get it to appear in Platinum you will need to go talk to Professor Oak and go to his house and the birds will start roaming around.
Moltres: Go to Cinnabar, beat Blaine and Bill will ask you if you want to go to One Island at which point you can go to Mt. Ember and climb up the mountain to get to Moltres, it is not available in Diamond or Pearl so you'll need to migrate one over and while it is available in Platinum you can still migrate one over, you'll need to talk to Professor Oak in order to have Moltres to start roaming in the region.
Mewtwo: had to the Cerulean Cave in FireRed, LeafGreen, HeartGold and SoulSilver after you've defeated the Champion and in the case of FireRed and LeafGreen after you have completed the Network Machine on One Island.
Mew: you can only get it in-game in Pokemon Emerald if you had received a ticket to get to Farway Island otherwise you can either use My Pokemon Ranch to receive one or you can use a cheating device in order to get it.
Raikou: For FireRed and LeafGreen it will start roaming around the region once you've beaten the Champion and have received the National Dex if Squirtle was your starter and it will roam around. also in HeartGold and SoulSilver
Entei: For FireRed and LeafGreen it will start roaming around the region once you've beaten the Champion and have received the National Dex if Bulbasaur was your starter and it will also roam around in HeartGold and SoulSilver.
Suicune: For FireRed and LeafGreen it will start roaming around the region once you've beaten the Champion and have received the National Dex if Charmander was your starter and for HeartGold and SoulSilver you will need to track it down around the Johto and Kanto regions until it stops by Bill's House in the Kanto region.
For Ho-Oh and Lugia, in FireRed, LeafGreen and Emerald you can go to Navel Rock by either using the event item or by using a cheating device like Action Replay to get the item to head there, both are available in HeartGold and SoulSilver and Ho-Oh is available in Colosseum for the GameCube and Lugia is available in XD for the GameCube.
Celebi was only available via an event distribution.
Regice: Get Wailord and Relicanth go to Pacifidlog Town on R/S/E and go left to the hidden chamber Wailord first Relicanth last and use Dig to go to the Regis chamber and for Platinum you will need to get the event Regigigas and then go to Mt. Coronet and right by the exit to Route 216 there will be a new cave which you'll go into it and you'll need to step on all of the dots and then interact with the statue and then you will fight the Level 30 Pokemon.
Registeel: Get Wailord and Relicanth go to Pacifidlog Town on R/S/E and go left to the hidden chamber Wailord first Relicanth last and use Dig go to the Regis chamber and for Platinum you will need to get the event Regigigas and then go to Iron Island and right by the exit there's a cave and if you go in there you will enter a new cave and once you interact with the statue a Level 30 Registeel will attack.
Regirock: Get Wailord and Relicanth go to Pacifidlog Town on R/S/E and go left to the hidden chamber Wailord first Relicanth last and use Dig go to the Regis chambers and for Platinum you will need to get the event Regigigas and then head to a cave in Route 228 and once you drop down you will be able to head into the newly-opened up cave and you will need to step on all of the dots on the floor before you can interact with the statue so that it will attack. Regirock is at Level 30 when you fight him.
Latios and Latios = in Ruby Latias will be the roaming one and Latios will be the roaming one in Sapphire and the opposite one will be the one that's available on Southern Island which you could only access either via the e-Reader and a special card or via a cheating device like an Action Replay. For Emerald you can pick which one you want to roam and which one will be the one that's on Southern Island.
Groudon: in Ruby go in the chamber in the white volcano city and Emerald go to the Weather Institute and the guy in the back will tell you there is a drough t somewhere it wont stay there forever and it has about 5 locations.
Kyogre: in Sapphire go in the chamber in the white volcano city and Emerald go to the Weather Institute and the guy in the back will tell you there is Heavy rain some where it wont stay there forever and it has about 5 locations.
Rayquaza: Go to Its tower near Pacifidlog Town and you will need the Mach Bike to get through.
Deoxys: For FireRed, LeafGreen and Emerald you could go to Birth Island via an event item however you could use an Action Replay to get the item in your game additionally with other games it would either be available via a Nintendo event or you needed to use an Action Replay in order to get it in your game.
Jirachi: Get from Colosseum bonus disc.
Uxie: Lake Acuity after solving the Dialga/Palkia problem
Mesprit: After solving the Dialga/Palkia problem you can head to Lake Verity, go into the cavern, interact with it and it will start to roam around the region.
Azelf: Go to Lake Verity after solving the Dialga/Palkia problem
Dialga: Spear Pillar climax in Pokemon Diamond
Paklia ;Spear Pillar climax in Pokemon Ppearl
Heatran ; Go to Stark Mountain a second time after going with Buck
Regigigas: Go to Snowpoint Temple with the three Regis in your party.
Giratina : Go to turn back cave, Defog and Rock Climb needed below Veilstone city, go to the Distortion World (Distortion World is only available in Platinum) after beating the Elite 4 to get the Griseous Orb to keep it in its Origin Forme.
Cresselia: After getting the National Dex you will need to talk to the Sailor's family in Canalave City and then the Sailor will take you to Fullmoon Island, travel around the island until you see Cresselia, interact with her and she will run it which will start the process of her roaming around the region.
Phione: Breed Manaphy with Ditto or you could use the Action Replay modifier code.
Manahy: Get 1 from Pokemon Ranger or use Action Replay.
Darkrai: Nintendo event or use Action Replay to get the item that's needed so you can enter into the locked place in Canalave City.
Shaymin: Nintendo event or use Action Replay to get the item needed and then go to route 224.
Arceus: Nintendo event or use Action Replay to get the item needed and head to Spear Pillar.
It is a rare pokemon, but it is not a legendary pokemon
Unknown is not a legendary Pokemon but it is considered rare. Legendary Pokemon are one of a kinds.
no lileep is not a legendary pokemon but it is an ancient pokemon
The legendary is palkia
No, snover is not a Legendary Pokemon
Azelf is a legendary Pokemon.
No, Blastoise is not a legendary pokemon.
Noivern is not a Legendary Pokemon.
Is sawk a legendary pokemon??
Groudon, Kyogre, and Rayquaza. Groudon is the fire legendary pokemon, Kyogre is the water legendary pokemon, and Rayquaza is the air legendary pokemon.
Arcanine is not a legendary Pokemon because legendary Pokemon can't be breeded and arcanine can.
It is absolutely impossible to get Legendary Pokemon eggs.