There are some ways to get Fennec in Rocket League 2021
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There’s a possibility that you may get a random blueprint at the end as a reward when you play a Rocket League match. The percentages are unclear, but there is a probability that Fennec may belong to that blueprint.
Blueprints allow you to craft the item they showcase, and in Rocket League, it is the cheapest way to unlock cosmetics. When you craft Fennec yourself, you can spend considerably fewer credits, but it will take longer than you would like.
Blueprints are untradeable Rocket League Items that only drop after online matches are done, so you would not be able to buy them on the internet.