These are pre-tuned cars by the Horizon team, aside from the visual modifications and performance modifications, they all come with an unique bonus that boost your skill points, XP or even cash.
As of right now they're not available from the autoshow, the only way to get one is from the wheelspins, or buying one out of the auction house.
Another thing to be noted is that not all cars are available in the wheelspins as of right now, only 7 out of the 33 cars are, these change from time to time where the current cars get removed from the wheelspin pool, and different ones are added in it's place.
Here's the list of all HE cars and their unique bonuses (courtesy from ManteoMax):
History of available cars:[09/23/2016 - 10/14/2016]-'14 BMW M4 Horizon Edition
-'97 BMW M3 (E36) Horizon Edition
-'69 Chevrolet Camaro Super Sport Coupe Horizon Edition
-'08 Dodge Viper ACR Horizon Edition
-'17 Ford GT Horizon Edition
-'17 Ford Focus RS Horizon Edition
-'69 Ford Mustang Boss 302 Horizon Edition
-'16 Jaguar F-Type Project 7 Horizion Edition
-'14 Lamborghini Huracan LP 610-4 Horizon Edition
-'12 Nissan GT-R Black Edition R35 Horizon Edition
-'13 Subaru BRZ Horizon Edition
[10/14/2016 - Currently Available]-'14 BAC Mono Horizon Edition
-'97 BMW M3 (E36) Horizon Edition
-'15 Chevrolet Corvette Z06 Horizon Edition
-'08 Dodge Viper ACR Horizon Edition
-'17 Ford GT Horizon Edition
-'17 Ford Focus RS Horizon Edition
-'88 Lamborghini Countach LP5000 QV Horizon Edition
-'12 Mercedes C63 AMG Coupe Black Series Horizon Edition
-'86 MG Metro 6R4 Horizon Edition
-'65 MINI Cooper S Horizon Edition
-'98 Subaru Impreza 22B STi Horizon Edition
Forzathon and other prizes:[10/28/2016 - 10/31/2016] - ForzaRC Week 1 Participation Prize-'17 Ford GT Horizon Edition
[11/04/2016 - 11/06/2016] - #Forzathon Prize-'17 Ford F-150 Raptor Horizon Edition
[11/06/2016 - 11/13/2016] - ForzaRC Week 3 Participation Prize-'16 Jaguar F-Type Project 7 Horizion Edition
[11/18/2016 - 11/24/2016] - #Forzathon Prize-'80 Subaru BRAT GL Horizon Edition
[11/27/2016 - 12/04/2016] - ForzaRC Week 6 Participation Prize-'12 Nissan GT-R Black Edition R35 Horizon Edition
[12/13/2016 - Currently Available] - Blizzard Mountain Completion Prize (160/160 stars required)-'13 Ram Runner Horizon Edition