Games 
Game Boy Advance Pokemon Emerald Legendary Pokemon Legendary Pokemon
Groudon and Kyogre
Groudon and Kyogre are ancient Pokemon and are very strong like Lugia and Ho-oh. Now you can capture both of them in the same game. After becoming the Champion in the Hoenn Region talk to the scientist inside the Weather Institute. For Kyogre he should mention something about a downpour in one of the following locations: Route 105, 125, 127 or 128. For Groudon he should mention a drought in one of the following locations: Route 114, 115, 116 or 118. For each route there are 2 possible entrances. Go to the route he said. If he said something about a downpour then rain should be falling but if he mentioned drought, it should be very sunny. Make sure you don't waste time because Groudon/Kyogre might escape and you'll start over again.
Terra Cave
If it's sunny and you're on the route the professor said, look for a cave opening, which is not usually on that route. Make sure you're fast because the cave will disappear from that route if you're not. Head into the cave and into another cave. The second cave you're in should be filled with mist; sort of like the Cave of Origin. At the end of the cave you'll find a LV70 Groudon. Defeat or capture it and the drought will end. Don't worry if you accidentally make Groudon faint, as you can have another chance to capture it by talking to the scientist at the Weather Institute again.
Click the maps below to see where the possible entrances to the cave are.
Route 114
Route 115
Route 116
Route 118
Marine Cave
If it's raining and you're on the route the professor said, look for a dive spot, which is not usually on that route. Make sure you're fast because the dive spot will disappear from the route if you're not. Dive into the dive spot and head into the cave. Head into another cave and you should be in a misty cave. Head to the end and you should find a LV70 Kyogre. Defeat or capture it and the downpour will end. Don't worry if you accidentally make Kyogre faint, as you can have another chance to capture it by talking to the scientist at the Weather Institute again.
Click the maps below to see where the possible entrances to the cave are.
Route 105
Route 125
Route 127
Route 129
Rayquaza
Rayquaza (LV70) as usual will be found on top Sky Tower. The first time you go to Sky Tower, you actually sort of ask Rayquaza for some help. You can actually catch Rayquaza after completing the game.
Regirock, Regice, Registeel
Unlocking the three legendaries is different in Pokemon Emerald. To access these three legendaries you have to first find a Relicanth (they can be caught underwater on Routes 124 and 126) and Wailord (either evolve a Wailmer or fish with the Super Rod on Route 129). Be sure to have them on your team along side Pokemon who can perform Dig, Surf, and Dive.
Now fly to Pacifidlog Town and perform surf, travelling west through the fast-moving currents. Be sure to remain close to the rocks on the south-side of the route. If you were successful, you should now be at a calm patch of water. Perform dive and follow the underwater passage. You should arrive at a Braille inscription. Follow the instructions and surface; you'll now be inside of a cave.
Once you're at the back of the cave, go to the wall and read the Braille message. Have your Pokemon perform Dig to cause an entrance to open. In the next room, make sure Wailord is the first Pokemon on your team, while Relicanth is last. After reading the Braille message at the back, an earthquake will cause the doors to the three hidden caves throughout Hoenn to open where the three legendaries dwell.
Regirock
Regirock is found where it usually is in the ancient tomb in the desert. Make sure you've opened the cave though in the underwater spot on Route 134. Once you're in the cave, walk up to the braille on the wall and the read it. Then take two steps left before taking two steps down. Then use Rock Smash and an opening should appear. Walk inside the cave and you'll see Regirock at LV40.
Regice
Regice is found in the Island Cave on Route 104. Make sure you've opened the cave just like you had to for Regirock. Once inside Island Cave, walk up and read the braille on the wall. Run around the walls of the room and when you reach the braille again, there should be a new opening. Make sure you run around exactly because if you don't, there won't be an opening. Walk into the opening and you should find Regice at LV40.
Registeel
Like the previous two Regis, make sure you've opened the tombs. Registeel will be in the Ancient Tomb on Route 120. Walk inside and read the braille. Walk into the middle of the room and use Flash. An opening should appear. Walk inside and you should find a LV40 Registeel.
Latias and Latios
After completing the game, you'll be back in your bedroom. Walk out and your dad will be there. Once he leaves, a news report will appear on TV. It will mention a colored Pokemon. Once the report ends, your mother will come over and ask you what color the Pokemon was. You actually get to choose between red or blue. If you choose red Latias will be wild in Hoenn but if you choose blue, Latios will be wild in Hoenn. Like with the legendary dogs, you have to chase them down if you want to catch them. The other Pokemon is found on Southern Island.
Lugia and Ho-oh
These two are found on Naval Rock. Naval Rock can only be reached with the Mystic Ticket, which is only obtainable at Nintendo events. Once you enter the cave, you'll see a ladder heading up and down. If you keep going down, you'll eventually reach Lugia. If you go up, you'll eventually reach Ho-oh. They are both at LV70.
Mew
Mew is found on Faraway Island. This place can only be reached with the Old Sea Map, which can only be obtained at Nintendo Events. Once you have it, just show the ticket to sailor in Lilycove Harbor. Once you reach Faraway Island, head into the forest. There, you'll find Mew. Mew will spot you and dive into the grass. You have to chase after Mew and corner it. If you do this, you can battle Mew. It'll be at LV30.
Jirachi
Jirachi can only be obtained at a Nintendo event. It is found nowhere else. There are rumors that you can travel into space to capture it but, they're wrong.
Deoxys
Deoxys is found on Birth Island. It can only be reached with the Aurora Ticket, which is only obtainable at Nintendo Events. Once you have the ticket, head to Lilycove Port and show the ticket to the sailor to be taken to Birth Island. Birth Island is small. You'll find a small triangle in the middle. That triangle is Deoxys. To solve the puzzle, touch the triangle to the nearest sides. Once you've completed the puzzle, you'll face a LV30 Deoxys. Deoxys is now in a new form, speed form, and like its name suggests, it's fast. This version can only be found in Pokemon Emerald. Make sure you capture it because this will be your only chance.
