So I have a bear-dog animal companion with only a bite natural attack, then she adds 1.5 times her Strength modifier to damage rolls with it. She has Power Attack both for extra damage and as a prerequisite for other combat feats (oh, right : she also has the very convenient Clever Critter feat to no longer be limited to a small list of basic feats).
Power Attack gives a bigger damage bonus to two-handed weapon attacks and primary natural weapons that give 1.5 times your Stength modifier to damage rolls. But some feats that require Power Attack sometimes mention a greater benefit for attacks performed with two hands but not primary natural weapons.
Examples :
Prerequisites: Str 13, Power Attack, base attack bonus +1.
Benefit: When you are wielding a two-handed weapon or a one-handed
weapon with two hands, and using the Power Attack feat, you do not
suffer Power Attack’s penalty on melee attack rolls on the first
attack you make each turn. You still suffer the penalty on any
additional attacks, including attacks of opportunity.
Prerequisite: Str 13, Power Attack, base attack bonus +1.
Benefit: When you are not mounted and you use the charge action, a
successful attack deals an additional 2d4 points of damage. If you are
using a 2-handed weapon, or a 1-handed weapon in 2 hands, you instead
deal an additional, 3d4 points of damage.
Prerequisites: Str 15, Power Attack, base attack bonus +1.
Benefit: When you hit a creature your size or smaller with a
two-handed weapon attack modified by the Power Attack feat, you can
choose to push the target 5 feet directly away from you instead of
dealing the extra damage from Power Attack. If you score a critical
hit, you can instead push the target 10 feet directly away from you.
This movement does not provoke attacks of opportunities, and the
target must end this move in a safe space it can stand in. You choose
which effect to apply after the attack roll has been made, but before
the damage is rolled.
Then would my bear-dog get the benefit of two-handed attacks with her bite with those feats, or only two-handed weapons benefit from them ?
Best Answer
A creature benefits more from those feats only when wielding either a one-handed melee weapon in two hands or a two-handed melee weapon; a typical creature's natural weapons are light weapons
The Pathfinder Design Team account—"the official Paizo account for making rulings for the Pathfinder RPG"—in a 2013 post made the following ruling: "Natural attacks are light weapons (though they are never expressly defined as such in the rules)." This lines up with the feat Weapon Finesse's Special entry that says, "Natural weapons are considered light weapons."
Thus, even were the bear-dog to somehow wield its mighty jaw in two hands (a task that is, so far as I'm aware, impossible), "[u]sing two hands to wield a light weapon gives no advantage on damage; the Strength bonus applies as though the weapon were held in the wielder's primary hand only," according to Equipment on Weapons on Light, One-Handed, and Two-Handed Melee Weapons, this despite a one-handed weapon wielded in two hands giving an advantage on damage.
The exception that increases the damage of a creature's lone natural attack ("If a creature has only one natural attack, it is always made using the creature’s full base attack bonus and adds 1-1/2 times the creature’s Strength bonus on damage rolls") does not change the creature's natural attack from a light weapon into a two-handed weapon.