As JC has openly acknowledge his love of low hanging fruit (or in this case meat) as well as a desire to up the danger level of darkness, I propose that deer (an most other non-monster animals) be given tiers by the simple act of
* slightly enlarging the model
* re-skinning it a noticeably different color shade
* buffing the stats such as HP, damage and speed
* increasing the 'aggressiveness' of the AI where possible
* upping the rare materials drop rate, meat quantity and purity/quality
The new more 'Dire' animal versions spawn in successively darker layers of darkness in place of the current animal, possibly with overlapping ranges if this is possible as it would expose a hunter to a greater mix of animals and he would need to be aware of the difference should be be able to take down one but not the other.
For the marque animals Deer the progression would look something like
Whitetail Deer
Mule Deer
Caribou
Elk
Moose
All animals continue to produce Venison so we don't overload food systems with excessive meat types, some things that rare drop on low level deer could drop consistently on higher tiers or their could be whole new drops that appear only at high levels. The difficulty increase from one tier to the next is comparable to the jump between existing animals like moving from Beaver to Deer hunting, An elite man-eating Beaver could be stronger then a normal Bear, but then their are Bear progressions too and the uber 'Cave Bear' will have maybe 500 blood or some ungodly amount.
But these are all still 'animals' and not monsters. I've always felt that their should be a meaningful distinction between animal and monster and the best distinction I can think of is that animals will KO you, but MONSTERS KEEEELLL YOU. To be Hunter of animals is one thing, to be a Monster hunter is a whole nother level of ballsy.