The Protector is a useful item if you want to evolve Rhydon, Rhyperior into Pokémon Sword and Shield. You have to go out of your way to get it, though, and it's a bit out of the way.
You can only purchase this item after you have upgraded your Rotom bike to cross the water. When you have it, you advance in the area needed to get it.
You want to go to Route 9 and proceed to the Circhester Bay location. You can reach it between the Route 9 tunnel and the town of Spikemuth. It is down a small elevated piece of land, right next to the sign. Head down the path and when you reach the bottom proceed through the water on your Rotom bike.
You want to go to the right side, through the jagged rocks. Cross them and proceed north, bypassing the coach on the small piece of land standing next to a large sign. Go around the small coach island to the right where you see blocks of ice forming. Proceed in this direction, staying at the top of the area to prevent Pokémon from wandering around the location.
Not long after, you should see a Poké Ball to the right of a green sign on a small island. Grab it and you will have The Protector, which you can give to Rhydon.