Free Cars via the Live Marketplace - The List
The following list of Free car unlocks (you still have to download them and then BUY them in game if you actually want them) is organized by year, manufacturer, and model, with a note about where they come from.
The Horizon Car List Section of the Guide will illustrate for you the wide selection of both makes and models of car that are available to the player in Forza: Horizon -- there is needless to say an incredible variety to be sure.
The acquisition of cars in Horizon is handled in a uniform fashion regardless of the source -- free or paid -- and that is that all of the cars that are actually available to the player (when all other qualifications are met) appear as a listing inside the Auto Show Tent at the Horizon Festival Hub, where the player can select them and then pay the purchase price (using in-game currency) whereupon that car is added to the players personal garage and inventory.
Once a car is purchased and added to the player's garage, they can customize it via the Paint Shop, modify it via Dax's Garage, and use it for racing (assuming that the car qualifies for whatever race the player is racing)...
All of the cars for the Base Game are automatically added to the Auto Show catalog and are available for purchase (assuming you have sufficient funds available for purchasing them.
Barn Finds: Cars that are obtained via the Barn Finds System are first hauled to Dax's Garage where he restores them, and upon the completion of the restoration process Dax sends the player a text indicating that to be the case.
To actually add that car to your garage you must visit Dax's Garage and "claim" the car from the Barn Finds Menu, but you don't actually have to purchase it from the catalog...
Adding New Cars via Download or DLC
If you own a Season Pass you can download the monthly car packs for free (OK it is not really free since you paid for the Season Pass but still...
The good news is that even if you decided not to buy the Season Pass you can, if you want, purchase the monthly car packs as a separate purchase.
Even better news, one car is actually selected from each of the monthly car packs and made available as a free unlock and addition to the Auto Show -- but that does mean that you must access the DLC section on the Live Marketplace, select the car listed as DLC, and download it first, and THEN you need to actually purchase that car via the Auto Show.
Some of the cars that are contained in the Monthly Packs are worth owning (jusdt saying) and to help you out in that respect what follows below is a list of all of the cars that you can obtain (download) via the Marketplace, and purchase in game (should you choose to)!
Notes and Other Information or Observations:
Some of these cars may be better models for certain types of races, and some of these cars may (or may not) apply to the 1,000 Club or the Rally Expansion Races -- but even if they don't free is good mates, so you may as well download them anyway :)
Also note that when you download the car from the Live Marketplace it is actually downloading and installing the data files for the car and integrating them into the game, and not just downloading a minor license stub. That being the case, be sure you have some free space on your storage device if you are using a USB stick -- that applies specifically to players who have the arcade version of the Xbox 360.
Carrot
Banana
What cars are included in the season pass
Marketplace