That is incorrect. A unicorn car is one that is very rare, and extremely valuable- like the ficticious creature (so they say, but ive seen a unicorn >.>...). The only people to be able to give them out are Turn 10 staff, and the only ways to get one are through winning an official Turn 10 competition (photomode, tuning, etc, they have a few different types running on their forums), they also give them out through Twitter and Facebook in trivia competitions, or you could go the expensive way and try to buy one from them through the auction house, but I really do mean expensive... Once you have one, you cant sell it on the auction house or gift it to anyone, but I do believe, if you want (cant see why...) you can sell it back to the game. As far as I know there hasnt been an official list of unicorns for FM4, but the 3 from FM3 are unicorns again (RX7 Spirit R, WRX S204, 69 Camaro SS), and I know that the old Gallardo Superleggera is one, as well as the 430 Scuderia. Its rumoured/speculated/assumed that the Mine's R32 and R34 Skylines are also unicorns. If my memory serves me correctly, there are 14 cars that are unicorns, but I cant remember the rest.
/didnt realize I was such a Forza nerd.