It was only open through the Totally Awesome Crates from xingwang's blog

Fennec and Octane are the two vehicles that are basically undefined from their hitbox shapes, making them two top choices for capable players. While Octane will be open for all players since it's one of the essential opened vehicles in Rocket League, you'll need to make a cursory Rocket League Credits
 effort to open Fennec.

 It was only open through the Totally Awesome Crates when it was first familiar with Rocket League, at this point with holders out of the game, you'll need to assess substitute ways to deal with open Fennec.

Rocket League has a solid trading structure www.lolga.com that licenses customers to share beautifiers or even sell them on pariah locales. The most easy way to deal with get yourself a Fennec will be through trading. Make a couple of requests your allies to check whether they have an additional copy of a Fennec, or you can look online to get yourself a humble type of the vehicle.