It took me a while to find my way into the doppeldrek. (Some monsters instantly have a theme or a hook or something that make them stand out; some take a bit more work. Ironically, shapechangers are sometimes the most trouble—once you get past the shapechanging, it’s not always clear what makes them special.)
Then I realized the answer was sci-fi TV (with a healthy dose of fantasy novels on the side). There’s always that episode (or chapter) where the hero winds up trapped in a cave or a starship with a shapeshifting goo. Worse yet, the longer the goo spends time in another shape, the more it tends to get ideas above its squicky station. After that, it’s usually a race against time to escape the cave or space the goo before it gets all murdery.
The doppeldrek seems a bit more innocent than that…it seems to just want to belong…but once it gets an intelligence score, well, all kinds of things can happen.
A game warden notices something odd about the deer in the King’s Wood. Some of them are acting oddly…just not quite right… When he sees one of them nearly gore a local lord, then collapse into foam on the end of his spear, he knows something his wrong. He hopes to hire adventurers to investigate quietly so he will not be punished for dereliction in his duties.
The elves didn’t leave this world. They just hid in it, using powerful enchantments to tuck their vast domains inside much smaller forests. These same enchantments also generate various guardians to protect the elfwoods, including turning river foam and fog into animate protectors. The misty oozes typically take the shape of stags, horses, wolves, and gars that work to drive away intruders. But as the elves become more insular and xenophobic, their guardians have begun to take the form of humanoids—even other elves—and sometimes exhibit a malign intelligence to boot.
A crashed kasatha starship holds a mimic sphere—a floating ball of ooze that spawns doppeldreks, who then take the shape of those they come in contact with. The dreks seem to operate independently of the sphere, and the few surviving kasathas know little about them. So where did the sphere come from? Are the doppeldreks part of it…its servants…or entirely separate entities? And what happens when they go from dumb miming to canny, self-aware mimicry in the course of only a few encounters?
—Pathfinder Bestiary 5 85