After you beat a local cyclist (Lvl? Sandslash); he tells you a rumor about a Pokeball in the darker part of the forest. No one knows what's inside it, but it has been there for a long time, and no one has been able to get it. His friend says if you try you'll need a special repel; he opens his backpack and gives you one. He warns you not to use it till the last second.
When you get to the area in Ilex Forest, you see a Spinarak, then another, then another. You quickly use the repel. A few more steps and you reach a clearing covered in hundreds Spinaraks and webs. Because of the repel they skitter only a few feet away from you. You spot the worn looking Pokeball tucked in the roots of a large tree. You reach for it...
A giant Ariados crawls out of the tree and stops inches from your face. The repel is too weak to affect it. The locals never anticipated one of them evolving, and to such massive size. You have to battle it, but you have to finish the fight in 4 moves. Before the repel wears off and the rest of them overwhelm you. If you manage to defeat the Ariados in 4 moves, you will have just enough time left to grab the Pokeball and run.
Inside the Pokeball is a level 10 Shellder with the moves Tackle, Withdraw, Supersonic, and (if your posts are 400 words or more) an egg move.
EDIT: I chose Shellder because there is no place you can get it. Except maybe an estate.