Beach Buggy Racing 2: Island Adventure (UP)

There are seven Hot Wheels cars in the game.

Rip Rod - Unlocked by default
Surf 'N Turf - Unlocked by default
Donut Drifter - Earn 3 Stars in Donut Drifter Cup in Village
Loco Punk - Earn 3 Stars in Loco Punk Cup in Marsh
Hotweiler - Earn 3 Stars in Hotweiler Cup in Jungle
Bone Shaker - Earn 3 Stars in Bone Shaker Cup in Desert
Twin Mill III - Earn 3 Stars in Twin Mill III Cup in Ice

You can unlock these on any difficulty.