Which Pokemon learn thief?

Teach it to a Butterfree with the ability Compundeyes. Compundeyes boosts the number of Pokemon holding items by 50%. Also teach it Sleep Powder, Whirlwind, and Silver Wind/ Bug Buzz. Use Sleep Powder to make the wild Pokemon sleep, then use Thief to steal the Item. If you cant escape, use whirlwind. Silver Wind/ Bug Buzz are just moves to protect Butterfree. You might consider putting a fainted Pokemon with the Frisk ability in the front slot ( Stantler, Shuppet, or Banette). Frisk lets you know if it has an Item or not.