This alone would give more incentive to using armor and would also make the kills more valuable.
Why? because animals in packs combined with a major hit points boost would be difficult to take down. It would take a hunting party.
That wouldn't be to say that no lone animals exist in the world, I just think it would be MUCH better if a majority of them traveled in packs (and double or triple all of the mobs hp....more even...i can 2 shot the highest HP mob in the game and DDT can 1 shot it).