North Americans have two names for Rangifer tarandus: reindeer and caribou. They are the same species but different subspecies. And humans had everything to do with that.
"Reindeer and caribou share the same genus, Rangifer, and species name, tarandus. Domestication is the biggest difference between reindeer and caribou. Reindeer are a semidomesticated subspecies of Rangifer, and there are many subspecies of both reindeer and caribou in Alaska," writes University of Alaska.
Humans began to domesticate caribou around 2m000 years ago in Eurasia. While Europeans simply call both the wild and domesticated subspecies reindeer, North Americans have two different names for them.
The differences that have popped up due to domestication include that reindeer are shorter and stouter than caribou, and they have thicker fur. Reindeer are also more sedentary than caribou: While they still migrate within a grazing range, they don't make the famously long migrations of caribou. The breeding season of reindeer has also shifted to earlier in the year, starting as much as a month before caribou.