Throughout the game, you'll be able to interact with many Pokemon. While these are not Legendary Pokemon, this is the main way of obtaining these Pokemon.
How to unlock
When you are in the Resort Desert area, there will be statues that you notice scattered around the area. When you come near the entrance to Ancient Castle, Professor Juniper will then tell you that these Darmanitan have a special ability known as Daruma Mode. A RageCandyBar is needed to awaken them so you can battle them. They are all at level 35 and have no special moves.
Around Route 6 and Route 10, these Pokemon will be disguised as Poke Balls (which represent items in the overworld) much like Voltorb and Electrode are in certain areas in previous games. In Route 6, they're at level 20, while they're at level 30 on Route 10.
Much like Foongus, these are disguised as Poke Balls though these are only found in the darker grass in Route 10. These are at level 40 when you find them.
This is found in the basement of the Dreamyard after beating the Elite 4. It can only be found on Fridays and it will be found at level 50. It has the special ability, Telepathy.
You must have an event Celebi that has been traded to your game via the relocator in Castelia City to unlock Zorua. Once Celebi is in your party you need to find a pair of NPCs in a building along the right-most street. Speak to the boy and he will turn into Zorua and join your party. Note that you must have an empty slot in your party and at least one pokeball (type does not matter). Zorua will be level 10.
You must have an event Raikou, Entei, or Suicune that has been traded to your game via the relocator in Castelia City to unlock Zoroark. Once you've traded one of the Legendary Dogs and have it in the active party, you need to go to the Lostlorn Forest (head east out of Nimbasa City) where you can battle Zoroark. It will be using its Illusion ability to show itself as one of the dogs you didn't bring. It's at level 25 and has no special moves.
Volcarona is located in a part of the Ancient Castle that is unlocked after you get the National Dex. The makeup of the new area is very similar to that of Turnback Cave from Diamond, Pearl, and Platinum. It is at level 70 once you find it so be prepared for a tough battle.