If you are a fan of the Forza series, chances are that you have played Forza 3 or 4 on your console, and that you have a game save from either of those on your console. That is important for several reasons:
(1) There is a Forza loyalty-based Achievement that you unlock after you win your first wristband which is awarded specifically because you have a save from either Forza 3 or Forza 4 on your console;
(2) There are special cars awarded to you IF you have reached specific levels in either Forza 3 or Forza 4, though some restrictions apply.
Obviously the first part is a no-brainer, as it just requires a save with some game play for the previous titles, but the second bit is more complicated because you are, in essence, being rewarded specifically for your progress in the previous games - which is fine if you completed them and have high XP levels, but not so fine if you have not...
In the latter case you will want to not play Horizon until you improve your standings in the previous games or rather in one of them at least. Why? Simple - these are the loyalty rewards for doing so:
As you can see, by working your Driver Level to 50 in either game you will receive some very tasty reward cars! Why that is important has more to do with what car you want to be starting your game with - since once you get to the garage in the starter VW you can pick from any of the cars you possess in your garage - and having these to choose from? Hey, I would go with a Dodge Charger over a VW any day, but that's me. Just saying.
Note though that I did say you have to get to the garage using the default starter car before you can access any of the loyalty reward cars - I did say that!
How it Works
To actually obtain the loyalty cars (and have them added to your game/garage) you first have to play through the basic starting level of the game until you unlock your first wristband (the Yellow one), at which point you can then proceed to Dak's Garage at Race Central and select the Change Car option to trigger the special event that adds them to your game.
You will be asked if you want to add the free loyalty reward cars directly to your garage for you being such a totally loyal bitchin' awesome dude (or dudette - at SuperCheats we do NOT discriminate), and of course the correct answer to that is “Hell Yes!” except they don't actually give you the option to answer that way, so you can really only say “Yes.”
So yeah, once you have reached that point you get to add those awesome cars, and you will also unlock the previously mentioned loyalty Achievement that is called “May the Forza Be With You” (10G) You received Free Cars for being a loyal Forza fan!
WARNING: Do not screw that choice up. If you ended up saying “No” you will not get the cars, not unlock the Achievement, and you will not be able to without completely deleting the game save from your system and starting over, from scratch.
Besides the annoyance of that, you may also be putting at risk other bonus items you may have received either as special promotions for buying the game from a specific store (depending on where in the real world you happen to be), for pre-ordering the game, and special offers that you unlocked online previously, such as the rare livery car you could have gotten if you downloaded the free trial version of the game from Xbox LIVE prior to the release of the game.
In addition to that you should be aware that some of the cars you will receive in this manner are special cars in the game in the sense that this is the only way to obtain them without spending real world money, using tokens, or credits in the in-game store.
It's important for you to understand that the actual award of cars in Horizon are NOT actual transfers from Forza 3 or 4 to Horizon. That is to say you do not have to have owned the cars in those games in order to get them in Horizon - once the game verifies that you qualify for the reward, you get it, no transfer involved.
Remember earlier when I pointed out that some restrictions apply? Yeah, the cars that you receive as your loyalty rewards have specific restrictions placed upon them. First, they cannot be gifted to other players because they were given to you for free by the game - that specific restriction tends to apply uniformly with gift cars, and technically the reward cars are gift cars.
On the plus-side, your loyalty reward cars CAN be shared with your club, which is cool, though they are not going to be a source for money for you since they have the same basic limitations that the cars in the previous Forza titles have, which is to say that they are only worth 100cr when you try to sell them...
Can I get the achievement if I played horizon before forza 3 or 4?