Changeling

Changeling, mortal;

Supernatural creatures of the night sometimes sneak into mortal people's homes under cover of the dark and steal young children right out of the crib. These babies are then raised by its kidnappers in the world of the supernatural, sometimes it is a servant and sometimes it is just as loved as its adoptive siblings, and if they ever escape or are allowed back to mortal world they possess knowledge and skills rare among their birth people. These children are called changelings, or mountain-taken, or the spirited away. Those creatures that break into mortal's homes to steal their children are mysterious and elusive fairy creatures, or dark things of the underworld like trolls, hags or goblins.
There are two kinds of changelings, the fearie-based that have been raised by the fey creatures of the forest, and the troll-based that have been raised in darkness by mostly evil forces. These templates can be added to any humanoid except elves (who are on good terms with the fairy world) and goblinoids (who steals children themselves).

Fearie-based

+2 Dex +2 Cha
Nymph's kiss bonus feat
Speak Sylvan language
Low-light vision
+2 bonus to Knowledge (arcana), Knowledge (nature), Handle Animal and Spellcraft
Spell-like ability: Speak with Animals 1/day, as a 1st level sorcerer.
Level adjustment: +1

Troll-based

+2 Str +2 Con
Troll's stench bonus feat
Speak either Giant or Goblin
Darkvision 30 ft (or +30 ft if already having darkvision)
+2 bonus to Craft (armorsmith), Craft (weaponsmith), Hide and Knowledge (dungeoneering)
Spell-like ability: Magic Stone 1/day, as a 1st level sorcerer.
Level adjustment: +1
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Changeling, supernatural:

Sometimes the kidnapper leaves one of its own children in the bed disguised as a mortal child after taking a mortal infant. But what happens to the troll or fairy child who gets left with the human parents? Their mothers don't just leave them to their faith; the child has been taught a special kind of magic by its real parents so it now can take the form of a mortal child, becoming a true changeling. Should the changeling fall into any danger its parents will try to secure it, often taking it back while giving the mortal child back, but evil creatures might seek revenge on the mortal community and start hunting down the people that slayed its offspring.

Changeling feat:

Prerequisites: Race: Fey, troll, goblinoid, hag or outsider; Size: Tiny to Large; Skills: Disguise 1 and Knowledge (arcana) 1; Adoptive parents (any humanoid but elf and goblinoid)
Gain: Alternative form
The young changeling becomes able to shift between its original form and one of the same race as its adoptive parents. This ability works just as a Polymorph spell cast by a 8th level sorcerer (or of the changeling's adult HD if higher), except that it only allows the changeling to take the form of a child of its adoptive parents race, there is no time limit and it can change between its two forms at will as a standard action. This gives the changeling a +10 bonus to Disguise checks when impersonating a child of its adoptive parents' race. The alternative form will be broken if a Dispel Magic is cast upon the changeling, but the changeling can return to its alternative form the next round, and a True Seeing spell reveals the changeling's true form.
Adoptive parents
The changeling also learns the language of its adoptive parents and gains a +4 bonus on Charisma checks against characters of the adoptive parents' race (so if a goblin changeling have dwarves as its adoptive parents it would learn Dwarven and gain +4 in Charisma checks towards dwarves).

When the changeling grows older and increase in HD it does not lose its Changeling feat. It will still be able to change into its alternative form, only now it is an older version of it. Adult characters cannot take this feat (unless having chosen it as one's first feat) and a young creature who takes this feat and grows up usually serves as ambassadors or spies for their birth parents.

Example: The following changeling uses a young (1 HD) troll as a base creature

Troll changeling

Medium-size Giant

Hit dice: 1d8+1 (5 hp)
Initiative: +0
Speed: 30 ft
Armor class: 12 (+2 natural); in alternative form: 11 (+1 size)
Base attack/Grapple: +0/+2
Attacks: 2 Claws +2 melee 1d4+2
Bite +2 melee 1d4+2
In alternative form: Unarmed strike +2 melee 1d2+2 nonlethal, attack of opportunity
Space/reach: 5 ft/5 ft
Special Attacks: -
Special Qualities: Adoptive parents, Alternative form, Darkvision 90 ft, Scent
Saves: F +3, R +0, W -1
Abilities: Str 15, Dex 10, Con 12, Int 8, Wis 9, Cha 6
Skills: Disguise +0*, Knowledge (arcana) +0, Listen +1, Spot +1
Feats: Changeling
Environment: Cold mountains
Organization: Solitary
Challenge rating: ½
Treasure: Standard
Alignment: Chaotic Evil
Advancement: -
Level adjustment: By character class

Adoptive parents: The troll changeling has humans as its adoptive parents, giving him a +4 bonus to Charisma checks against humans. He also gains Common as a bonus language.

Alternative form: The troll changeling can change between its real form and that of a human child at will as a standard action. *While in his human form, the troll changeling gains a +10 bonus to Disguise checks when impersonating a human child.

What first seems to be a five year old human boy, with rugged and ugly features and being unusually big for his age, is actually a young troll in disguise. The troll changeling has been hiding in plain sight among its human parents for years while the child he is impersonating slaves away under the changeling's real parents. He uses his unusual strength to bully the other children around him, stealing, breaking into people's houses at night and killing pets and farm animals to sate his great hunger. He behaves oddly, often appearing more precocious and meaner than the other children of his age, but his adoptive parents and the rest of the town are unknowing of the child's true identity and see him just as a tearaway then an actual monster.

A troll changeling might serve as an early opponent to low-level characters and prepare them for adult trolls and harder enemies later on. A good quest for early adventurers would be to force the changeling to reveal where the actual child is held captive, and then go out to free him or her. Maybe they can trade the changeling in exchange him for the human child, or maybe they just slay him as a warning to other trolls. Remember that if the characters can't prove that the changeling isn't really human, the town's people might not like seeing them attack a defenseless child.
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Nymph's kiss feat

By maintaining a close relationship with a fey (such as a nymph or dryad), you gain some of the characteristics of fey.
Benefits: Fey creatures regard you as you were a fey. You gain a +2 circumstance bonus on all Charisma-related checks and a +1 bonus on all saving throws against spells and spell-like abilities. Starting with the level you take this feat, you gain 1 extra skill point per level.

Troll's stench feat

By living among the creatures of the dark underground and learning their ways, you gain some of the characteristics of goblins and trolls.
Benefits: Goblinoids and trolls regard you as one of their own. You gain a +1 bonus to all Fortitude saves and you also gain a +2 circumstance bonus to all Charisma-related checks towards goblinoids and trolls. Starting with the level you take this feat, you gain 1 bonus hit point per level.