Once, in 2014. And that image on the right is not how they looked.

[WhiskeyGrinder]

https://www.reuters.com/article/fact-check/finland-did-paint-some-reindeers-antlers-with-reflective-paint-but-social-medi-idUSL1N2OP2K9/

A digital artwork showing a reindeer with orange-glowing antlers has tricked users on social media into thinking it is a real photograph .
Most posts accompanying the image refer to real efforts in a region of Finland in 2014 to reduce reindeer road deaths by painting their antlers with reflective paint. The image accompanying the true story, however, is not a real photograph.