Apparently there is a new unannounced car in the carlist FH5. I just discovered it when i looke at the Horizon Promo.
So researched how to get the car. In a bunch of countries you can purchase Oreo-packages with some XBox-goodies/codes for Halo, FH5 and Sea of thieves. The FH5 goodie is the Pagani Zonda Cinque Roadster "Oreo Edition". Players who have the game on Steam have to buy a code somewhere, but not directly on Steam.
more detailed informations you find here
I have the game on Steam and i bought a code here for 1,49 Euro:
eneba.com