I'm not sure if this is official but here is the list of all the unicorns.
From
http://www.gtplanet.net/forum/showthread.php?t=213883
Forza 4 Unicorn cars
2002 M3-GTR "Street Version" [Unicorn / Buy in Auction House]
1969 Camaro SS Coupe [Unicorn / FM3 Profile Import if in FM3 Garage ~ Or buy in Auction House]
2007 430 Scuderia [Unicorn / Get via FM3 Profile Import if in FM3 Garage ~ Or buy in Auction House]
2003 Challenge Stradale [Unicorn / Buy in Auction House]
2007 Shelby GT500 [Unicorn / Buy in Auction House]
2004 Mugen Civic Type-R [Unicorn / Buy in Auction House]
2002 Mugen Integra Type-R [Unicorn / Buy in Auction House]
2005 NSX-R GT [Unicorn / Buy in Auction House]
2007 Gallardo Superleggera [Unicorn / Get via FM3 Profile Import if in FM3 Garage ~ Or buy in Auction House]
2007 Murciélago LP640 [Unicorn / Buy in Auction House]
2002 RX-7 Spirit R Type-A [Unicorn / FM3 Profile Import if in FM3 Garage ~ Or buy in Auction House]
2006 HKS Time Attack Evolution CT230R [Unicorn / FM3 Profile Import if in FM3 Garage ~ Or buy in Auction House]
1993 MINE'S R32 Skyline GT-R [Unicorn / Buy in Auction House]
2002 MINE'S R34 Skyline GT-R [Unicorn / Buy in Auction House]
2004 Top Secret Silvia D1-Spec S15 [image] [Unicorn / Buy in Auction House]
2006 Impreza S204 [Unicorn / FM3 Profile Import if in FM3 Garage ~ Or buy in Auction House]
2002 Top Secret 0-300 Supra [Unicorn / Buy in Auction House]