Ok, so in Pokemon White 2 (The game i play.), I located Ditto! And another Pokemon as well! In the game, you find where team Plasma's ship is sitting at. Where you finally battle Ghetsis, right? YUPP! That'd be the right place- but one problem, its all coated in ice! Once you beat Ghetsis, return to the Pokemon Center, heal up your Pokemon, Beat te E4 (Elite 4), and Champion Isis. Thats right, CHAMP ISIS!!!!! Prepare with dragon-type pokemon so you can beat her!
After that, you will enter whats called the "Hall of Fame." You will also see the Credits. Dont worry, the game will save (If yours says nothing about it saving, sorry, sit tight for the credits!) If it says would you like to save the game say yes and afterward, as it begins to show you the Credits, just turn off the game! Then, turn it back on and get your fare-well from Isis and the Elite Four!
You will return home and there your rival and his sister will give you valuable information about a Zoroark (The evolution of Zorua, whom you should have obtained in Driftveil City. Evolves at Level 30.), that is waiting for you on Victory Road. Chase this down, and you will be lead to N's Castle. He will then battle you using only his Dragon Pokemon. (Reshiram or Zekrom).
Anyways, after that LONG detour, you will come back to find all of the ice to have been rmoved (Beating N is neccesary at some point in the game but not before ice is gone). You will be able to find wild Ditto and Metang and Delibird and many others such as Piloswine! And Solrock and Lunatone as well. Also, the most ANNOYING Pokemon OF THEM ALL, Audino. Audino appears in that rustling grass in this area, as it does in many other places as well. Clefairy is also availabe here.
Pokeballs and Items to equip yourself with if you plan to head to this area: Great Balls, Poke Balls, Master Balls (If possible). Max Revives! Repells (For getting through the rock).
Hope this was Helpfull in SOME way.