Specifically, does a Quadruped eidolon add its STR, or 1.5*STR on its bite attacks?
Assume for the sake of this question that the eidolon has at least one other natural attack and that it's not using any weapons to attack.
The PFSRD describes the bite evolution like this (emphasis mine):
An eidolon’s maw is full of razor-sharp teeth, giving it a bite attack. This attack is a primary attack. The bite deals 1d6 points of damage (1d8 if Large, 2d6 if Huge). If the eidolon already has a bite attack, this evolution allows it to deal 1-1/2 times its Strength modifier on damage rolls made with its bite.
Earlier on that same page, we see that a quadruped eidolon gets the bite evolution for free. However, it also lists a bite attack in the base statistics. It seems like there are two possible cases:
- The eidolon has a bite attack because it naturally has sharp teeth. Additionally, the eidolon gets the bite evolution for free, which means its bite attack adds 1-1/2 times its Strength modifier on damage rolls.
- The eidolon receives the bite evolution for free, granting it a natural bite attack. Because the bite is a primary attack (but not the only natural attack, meaning it doesn't add 1-1/2 anyway), the eidolon's bite attack adds its Strength modifier on damage rolls.
Rules-As-Written is most preferable, but it seems as though the rules aren't clear, so a justification of Rules-As-Intended or even What-Makes-The-Most-Sense is appreciated. If you don't have a justification for one of these two possibilities, feel free to recommend your own interpretation or sensible house rule.