Within the Dream World, you'll occassionally encounter a variety of Pokémon. These Pokémon are not typically available within the Unova region and thus are mostly Pokémon that were previously native to Kanto, Johto, Hoenn & Sinnoh. These Pokémon, once you have befriended them, will then be able to be encountered within a specific area of Unova. In this area, you will be able to capture them with great ease.
Many of the Pokémon within the Dream World will have abilities that the Pokémon you transfer over from Diamond, Pearl, Platinum, HeartGold & SoulSilver don't normally have, such as Vaporeon with Hydration and Glaceon with Ice Body and some will have hold items that they don't normally have in the wild. Below is a list of all the hold items.