I think the criteria would be its potential to cause harm and/or death.
As an (dumb) example, poke a hole in a Sable at 40yards and it probably wont turn around and try stick its right horn up your a**, poke a hole in a lioness and she may turn around with a look that says "nice try, now its my turn sunshine"!
I tend to think the common DG that we know of fit that bill, although I would not like to get too close to an injured Sable, I still won't classify it as DG. Dangerous under a unique circumstance, yes, but all wild animals can be dangerous under unique circumstances.
The fact the Buff, Croc, Ele, Hippo, Leopard, Lion and Rhino are more capable of harming/killing under a wider variety of circumstances would make it DG in my opinion.
But that's my opinion only.
On a lighter note, maybe we can add Mosquito's and Mamba's to our list of DG??!??! :crazylol:
Funny you said that! The Mamba and Mosquito crossed my mind!