Deer is the family of ruminant mammals. Its species include, among others, the moose, red deer, reindeer, fallow deer and roe deer. They are natural food for wolves, bears, panthers and other predators. None of those has been domesticated yet; however some members of the elder races are able to ride them. 
Members of the human cultures tend to hunt deers for food and fame. They are though protected in some areas, like surrounding of druid circles or royal forests - where only the nobility have a permission to hunt.
- Most of the creatures in The Witcher 3: Wild Hunt Geralt can kill have an actual scent that attracts other monsters in the area. Meaning that scene where hungry bear walks into a pack of wolves busy chewing on a deer can occure.