This topic inspired me...
Need For Speed 2 SE, Jurassic Park style:
Holiday Racer, caravanning through Eau Rouge:
Moto Racer 3 bonus sci-fi bike; that's not gone well!:
Manx TT SuperBike (arcade) - the new TT game looks nice, but unless you can ride a sheep down Bray Hill... it's not the same!:
ToCA Race Driver 3 - 'mower drift:
Just Cause 3 - the racetrack in the sky:
Others I wanted to include, but failed to (due to games playing up, etc):
Crash Time IV - The Syndicate: golf buggy (not unlocked it).
Rally Championship 2000: The Turbotater potato car (game won't work for me at the moment).
Sega Rally 2: Kerolla WRC frog cartoon car (not sure how to unlock on PC).
Hydro Thunder: Tinytanic (racing in iceberg level) (not unlocked at the moment).
Daytona USA: bonus horse (it's too well-known. And too low-res (PC port!)).