There are a few that appear on wild Pokemon.
- Grimer and Muk can hold Black Sludge
- Munchlax and Snorlax will always be holding Leftovers
- Makuhita can hold an Expert Belt
- Bounsweet, Steenee, and Tsareena can hold a Grassy Seed
- Togedemaru can hold an Electric Seed
- Poliwhirl, Politoed, Poliwrath, Slowbro, Slowking, Hariyama, and Hawlucha can hold a King's Rock
- Lombre, Leavannty, Pancham, and Pangoro can hold a Mental Herb
- Comfey can hold a Misty Seed
- Nuzleaf can hold a Power Herb
- Scraggy, Goomy, and Sliggoo can hold a Shed Shell
Those are all the ones I could find. It does slightly depend on which items you consider "competitive" though. I mean, items can like the Absorb Bulb and the Cell Battery are also found on wild Pokemon, but I have never seen them used, so I don't really consider them a competitive item.
