“Unicorn cars” is what some people call the really expensive cars in the game, usually the ones that cost 10mil credits and up.
Aston Martin DB3S - 10mil
Alfa Romeo Giulia TZ2 - 10mil
Ferrari 250 GT - 12.5mil
Jaguar D-Type - 15mil
Shelby Daytona Coupe - 15mil
Lamborghini Miura P400 - 15mil
Ferrari 250 GTO - 20mil
Ferrari 330 P4 - 20mil
Jaguar XJ13 - 20mil
Ford GT Mark IV - 20mil
All except for the Ferrari 250 GT and Lambo Miura P400 can enter the Nostalgic 1979 event in the Professional League in Campaign mode. This event, specifically the race at La Sarthe, is one of the best credit farming races in the game.
Of all the cars that can enter the race, only the Alfa Romeo Giulia TZ2 is NOT capable of winning. All of the other cars can win the La Sarthe race. Some of them need max HP upgrades and max weight reduction upgrades, and softer tires, but they can all win with the clean race bonus.
I think the term comes from older GT games where some of these same cars used to only be available through the Used Car Dealer, and would only appear there very rarely. The rarity of them being in the UCD combined with a high price tag made it really rare to see these cars in online lobbies...when you saw one, it was a “unicorn”
. It’s still rare to see them in GTS online lobbies, but only because of the boring credit grind in GTS.