Changelings were fairies who had been left in place of a human child or baby who had been stolen by the fairies. The child was taken for one of three reasons; to act as a servant, for the fairies to receive the love of a human child, or for malice/revenge.
"A human child might be taken due to many factors: to act as a servant, the love of a human child, or malice. Most often it was thought that fairies exchanged the children.
According to legend, the abducted human children are given to the devil or used to strengthen fairy stock. The return of the original child may be effected by making the changeling laugh or by torturing it; this latter belief was responsible for numerous cases of actual child abuse.
The new child—the changeling—is characterised by unresponsiveness, resistance to physical affection, obstreperousness, inability to express emotion, and unexplained crying and physical changes such as rigidity and deformity. Some are unable to speak.
According to Thomas Keightley, fairies look for human infants with the intention of offloading any fairy children "which they foresee likely to be feeble in mind, in body, in beauty, or other gifts."
Halflings are a rare species that occurs when a faerie fraternizes with a human. These hybrids possess limited fae powers and have high attraction to and from the supernatural.
Sometimes changelings are old, nasty fairies who revel in the sorrow they cause; or fairies with prodigious appetites for human food or mortal breast milk. Sometimes the changelings are fairies so old and worn out that their kinfolk have left them behind, happy to be rid of them in exchange for a plump human child.
Changelings don't inherently recognize each other. If they don't want to be recognized then when they do something suspicious it would be an insight check against the changelings deception, which the changeling has advantage on. A changeling could have expertise in insight from divergent persona.
Changelings born from two changeling parents are born in their natural form: pale, slender, with colorless eyes and white hair.
Lifespan. Changelings reach maturity slightly faster than humans and live as long as humans do, typically about 100 years.
Most changelings don't know they're changelings.
Though they may always feel somewhat different or removed from human life, most changelings are too young to understand who they truly are.
The true form of a changeling is a bit paler than a typical human with slightly elongated limbs. Their eyes are colorless and ringed by black as pictured in the artwork above by Julia Zhuravleva. They have no body hair, but the hair on their head can be platinum, white, or silver.
Changelings are typically born with female anatomy, but some prefer to maintain male primary and secondary sex characteristics once in control of this ability.
By RAW changelings revert when they DIE but not when they are unconscious (it would be hard to maintain a long term masquerade if you revert when you sleep!).
Weaknesses. Connection to their mother – Killing a mother changeling will cause its children to die instantly. Fire – Setting a changeling on fire will kill it. Mirrors – A changeling's true form is revealed in a mirror.
In some cases, the changeling grows up often being very loyal and caring to friends and family despite what they are. They often care more for their friends and family than they do themselves. Most changelings enjoy helping as they get older, but they often feel different, out of place.
Obviously, a changeling's appearance can change at any moment, but they do have a true form. Their true skin is usually grey or pale white, and their facial features are blank slates to an unnerving degree. They're typically thin and gaunt, and their hair is usually silvery with an occasional green or pink tint.
according to the rukles for changelings: Languages: Changelings begin play speaking the primary language of their host society and Common. Changelings who have high Intelligence scores can choose from among the following bonus languages: Aklo, Draconic, Dwarven, Elven, Giant, Gnoll, Goblin, and Orc.
Changelings have red blood like a majority of the other races. Bleeding around others may alert many that the Changeling is not of the same race (should the blood color vary). For this reason, a Changeling will usually choose to take the form of a race with red blood.
A changeling can't make the wings at all. If they had wings, like from a class feature or something, then they could look like an Aaracokra with wings because they will have those limbs at that point.
As for being tattooed, changelings can definitely be tattooed, and they can just as easily erase the tattoo with a moment's thought.
Age. Changelings mature slightly faster than humans but share a similar lifespan—typically a century or less. While a changeling can transform to conceal their age, the effects of aging affect them similarly to humans.
Within this description there are variants of changelings that show subtle differences. Some have opalescence in their skin (as if it contains various colors instead of just being white) and their eyes can be completely black.
Humans altered by the Wyrd to become Changelings can eventually turn into True Fae themselves. Perhaps the very first True Fae was a human who stumbled into Arcadia, and had to become part of it to survive... then started bringing more humans in... As a corollary to the above.
Unlike ponies, changelings do not have cutie marks.