The Stock Car Races and Cheval Marshall monster truck are exclusive to returning players of Grand Theft Auto 5 on PS4 and Xbox One. A marker for the races will appear on your map after you complete the Mr. Philips mission with Trevor.
You'll need to find a suitable ride to enter the first race - a muscle or sports car should do it. Win races to unlock the following vehicles, in order:
1: Burger Shot Stallion
2: Redwood Gauntlet
3: Pißwasser Dominator
4: Sprunk Buffalo
5: Cheval Marshall
Once unlocked, you can access the four stock cars from any of the characters' personal garages.
You can also collect the Cheval Marshall from Trevor's airstrip hanger in Grand Senora Desert, with a set of 25 different flag liveries to choose from.
Looking for more help with Grand Theft Auto 5? Our complete walkthrough guide has got you covered.