What are Unicorn cars
These are cars, that you can't simply buy or win. To get them, you sometimes can buy them in Auction House (their prices easily reach multi-million count), win in Turn 10 competitions, import them from Forza Motorsport 3 save (if you bought them in that game) etc. There are 18 Unicorn Cars in Forza Motorsport 4, all listed below with explanation, how to get them.
Unicorn cars List
01. 2002 BMW M3-GTR "Street Version" [buy in Auction House]
02. 1969 Chevrolet Camaro SS Coupe [FM3 Profile Import if in FM3 Garage ~ Or buy in Auction House]
03. 2007 Ferrari 430 Scuderia [Get via FM3 Profile Import if in FM3 Garage ~ Or buy in Auction House]
04. 2003 Ferrari Challenge Stradale [buy in Auction House]
05. 2007 Ford Shelby GT500 [buy in Auction House]
06. 2004 Honda Mugen Civic Type-R [buy in Auction House]
07. 2002 Honda Mugen Integra Type-R [buy in Auction House]
08. 2005 Honda NSX-R GT [buy in Auction House]
09. 2007 Lamborghini Gallardo Superleggera [Get via FM3 Profile Import if in FM3 Garage ~ Or buy in Auction House]
10. 2007 Lamborghini Murciélago LP640 [buy in Auction House]
11. 2002 Mazda RX-7 Spirit R Type-A [FM3 Profile Import if in FM3 Garage ~ Or buy in Auction House]
12. 2006 Mitsubishi HKS Time Attack Evolution CT230R [buy in Auction House]
13. 1993 Nissan MINE'S R32 Skyline GT-R [buy in Auction House]
14. 2002 Nissan MINE'S R34 Skyline GT-R [buy in Auction House]
15. 2004 Nissan Top Secret Silvia D1-Spec S15 [buy in Auction House]
16. 2006 Subaru Impreza S204 [FM3 Profile Import if in FM3 Garage ~ Or buy in Auction House]
17. 2002 Toyota Top Secret 0-300 Supra [buy in Auction House]
18. NULL CAR (hidden unicorn car, looks like blacked out VW Jetta / Bora often seen when users do not have DLC content and see DLC CAR