The shifter class makes mention of a "natural form" as a prerequisite for gaining claws, presumably to restrict the ability to add claws to various Wild Shape forms.
The Skinwalker race has a Change Shape supernatural ability to assume a more bestial form. This is not a spell, nor does it mimic one (as Wild Shape does), and as a function of the race it is a "natural form", though it is possibly still considered a polymorph effect.
Can a Skinwalker Shifter in their bestial form sprout Shifter claws?
Change Shape (Su)A creature with this special quality has the ability to assume the appearance of a specific creature or type of creature (usually a humanoid), but retains most of its own physical qualities. A creature cannot change shape to a form more than one size category smaller or larger than its original form. This ability functions as a polymorph spell, the type of which is listed in the creature’s description, but the creature does not adjust its ability scores (although it gains any other abilities of the creature it mimics). Unless otherwise stated, it can remain in an alternate form indefinitely. Some creatures, such as lycanthropes, can transform into unique forms with special modifiers and abilities. These creatures do adjust their ability scores, as noted in their descriptions.