This page will list some of the confirmed Legendaries you can acquire in the full game. More details will be released once they are available.
Ho-Oh
Pokémon Gold Version and Pokémon HeartGold Version
Category: Rainbow Pokémon
Type: Fire/Flying
Ability: Pressure
Height: 12'06"
Weight: 438.7 lbs.
Ho-Oh is a Legendary Pokémon said to fly through the skies above the world with its fantastic rainbow-colored wings. People say that rainbows follow in Ho-Oh's path wherever it flies.
Lugia
Pokémon Silver Version and Pokémon SoulSilver Version
Category: Diving Pokémon
Type: Psychic/Flying
Ability: Pressure
Height: 17'01"
Weight: 476.2 lbs.
With tough wings and powerful stats, this Legendary Pokémon is heralded as a god of the seas. When Lugia flaps its wings, it whips up a storm said to last 40 days.
Dialga
Diamond Version
Category: Temporal Pokémon
Type: Steel/Dragon
Ability: Pressure
Height: 17'09"
Weight: 1,505.8 lbs.
Dialga is a Legendary Pokémon said to be able to move between the past and the future. It can freely manipulate the flow of time.
Palkia
Pearl Version
Category: Spatial Pokémon
Type: Water/Dragon
Ability: Pressure
Height: 13'09"
Weight: 740.8 lbs.
This Legendary Pokémon is said to be able to move between distant places and even different dimensions. It can freely manipulate the links between dimensions.
Reshiram
White Version
Category: Vast White Pokémon
Type: Dragon/Fire
Ability: Turboblaze
Height: 10'06"
Weight: 727.5 lbs.
The Legendary Pokémon Reshiram could burn all the world with its flames. It is said to come to the aid of those who want to build a world based on truth.
Zekrom
Black Version
Category: Deep Black Pokémon
Type: Dragon/Electric
Ability: Teravolt
Height: 9'06"
Weight: 760.6 lbs.
The Legendary Pokémon Zekrom could fry the whole world with its lightning. It is said to stand beside those who want to build a world based on ideals.
Reference and Image Resource: www.pokemonrubysapphire.com
How to find fabled cave
All of these mirage spot Pokemon are not shiny locked, meaning you may be able to get shiny versions if you soft reset enough...Good Luck!
The three spirits need one Pokemon with full happiness in your party
After completing the delta episode and holding at least one Pokemon in your party that is lv 100, soar around the mountains to find a mirage spot which oolds Zekrom(Alpha Sapphire) or Reshiram(Omega Ruby) hope this helped ;)
After obtaining the three lake spirits from nameless cave sour around and you can find a rip in the sky where DPs legends dwell.