In Pathfinder Primary Secondary and 1.5str are based on the Natural Weapon. If you only have a single attack it gets 1.5 str damage. For the list of Primary/Secondary attacks types look in the
Bestiary Section.
The only other way I know to "force" a type to be different is to use my
Adjustments addon or a script. Then you could use:
Natural Attack: Secondary; Natural Attack: Primary; Natural Attack: Two-Handed Damage adjustments which can change the type of a natural attack.