So animals range up to what, 6 str? With Combat 6 they have a 75% combat modifier. That's 6 promotions. Then there could be a few more... lets say they really develop it out and get up to about 150% combat modifier against your ranger.
That means the strongest possible animal could end up being about 15 total effective strength.
Compare to the Ranger, I believe is 5 str? (and we're getting nowhere near the game and big game hunters here who are far beyond this.) Plus, as ls612 points out, 300% BASE from simply being a ranger, against animals. Let's say that Ranger is completely non-promoted.
That means the Ranger, unpromoted at all, against a lvl over 7 animal would end up being a strength 20 (5 * 3 + 5).
At 5 pts higher than the strongest animal in a completely unpromoted state... I'm not too worried about giving animals some promotions. It's the human barbs that Rangers and such have to watch out for... they aren't nearly so well suited to fighting people with weapons.