While this is a little bit of a grey area, we will err on the side of caution and encourage people to stick with actual deer or beings that directly resemble deer. Once hybrids get involved it can be easy to move fairly far away from the original animal motif by sticking antlers on a completely different animal. That said, any creature in the cervidae family, or resembling a creature in the cervidae family, should be fine. Some allowances may be had; say for things like robotic deer, or mythical beings that are directly based on a deer design but jackalopes, like Fatigue, are unique creatures unto themselves. That said, hybrids are still welcome to play a role in the story! So long as there is at least one deer present.
Cervidae Family: https://en.wikipedia.org/wiki/List_of_cervids