I was going through the SRD & converting all the animals. The hardest parts are determing what feat to replace Weapon Finesse with (Weapon Focus if 2HD or larger, or else Toughness or Alertness or even a Skill Focus seem apropriate) or what value to assign AP. In the Conan Core book, Boars get 6 AP for Gores, Bears get 9 AP for Bites, and Sabre-toothes (-teeth?) get 12 AP for Bites. Since we mostly have a progression of 3's, I broke it down this way: Medium-sized Gores have AP 6, Large Gores or Bites- AP 9, Huge Gores or Bites AP 12. Any critter that uses finesse attacks loses the AP (therefore eliminating most snakes). A sabre-tooth has the equivalent of Improved Natural Attack, therefore qualifying it for the Huge AP. Looking through the SRD, that leaves Elephants & some fantastic animals (Kraken) with gore or bites of AP 12. Your average Lion, Bear, Tiger has a bite of AP 9. And then any medium-sized animal with a gore attack would get AP 6 but Boars are it.
Now, if you look athe Fantastical section of the Bestiary, all the Garagantuan Elementals get AP. Probably because it just doesn't matter if you're wearing Plate or Leather when they sit on you. Humanoid-like monsters (which I'm throwing in the Grey Ape from the animals section) may get some kind of AP for what may be a rending atack. Grey Apes gore at AP 4, Ghouls bite at AP 4. Therefore maybe being Medium or Large humanoid & having a more savage attack qualifies you for that AP 4.
Think this even close to how Mongoose figured it?