Was reviewing a new release from Lost Spheres Codex of Blood: Parasites & Paragons
. The new class in there can grab this ability at second level.
Alien Form (Ex): Choose a creature type other than your own. A parasite with this mutation is treated as this type rather than their original type whenever it would be beneficial (for example, harmful spells which target only humanoids such as Charm Person or Hold Person will automatically fail on a humanoid parasite who selected aberration with this ability unless they specifically chooses otherwise). A parasite can select this mutation any number of times, each time selecting a new type to be treated as.
Are there any particular uses for this ability, assuming it doesn't give immunities, proficiency, lack of bodily needs, or racial HD properties of the new type? I recall PF plugged the big advantages of counting as another type in 3.5 and can't think of anything particularly impressive, but I'm sure there's something interesting to do with it.