An animal has two hit values, the first is the number of hits to render unconscious, the second being the additional hits to kill.
For a 100kg animal this is 5D/2D.
I would say that when shooting an animal with a tranq round its save is based on its hits to kill.
So a 100kg pouncer with 14/7 hit with a tranq round would roll 7 or less to remain conscious.
Note for very large animals they may not need to roll since their hits to kill is 12 or greater, you will need a more powerful tranq round that is probably lethal to smaller animals/PCs