Road Hog Xl is a Import Body.
Bodies are the most essential customizable element of Rocket League. They all have different hit boxes so choose wisely which fits to your playstyle. You can equip them inside the customization window in the body tab.
To get the Road Hog Xl Body you can either open a specific crate with a key or trade with other players.
You can search for trade offers with Road Hog Xl here.