That is a Magmarizer. It is a possible wild hold item for Magby, and it is very difficult to come by. To find wild Magby, you need a LeafGreen cartridge. As long as the LeafGreen cartridge is in one of the slots, Magby will appear in the Stark Mountain area. Still, getting a Magmarizer is quite a chore. I have gone through more than 30 Magby, and have yet to obtain a Magmarizer. You may want to try putting a Pokémon with the Compound Eyes ability at the front of your team to hopefully improve your chances of obtaining a Magmarizer.
Also, to evolve, you must trade your Magmar while it is holding the Magmarizer.