Changeling

Changelings, sometimes called doppelgangers, may resemble humanoids but are actually related to mimics. They are essentially a sentient blob of grey goo. They are able to mimic the appearance of any humanoid and any other 4 limbed creature of humanoid size.

They are brood parasites. Changelings can sense and seek out other changelings. They exchange genetic material and form an egg. One of the changelings then takes the egg and finds a mating pair of humanoids. They will take on the appearance of the male and simulate mating with the female, implanting the egg into their womb. The changeling foetus will develop inside the humanoid’s womb and grow to resemble the host humanoid. From birth, they will maintain this form and believe themselves a member of that species until they hit puberty. Once they reach this point, they gain the abilities and innate knowledge of a changeling.

Most choose to maintain their disguise until they leave home, but some are found out early or embrace their identity. This can often lead to rejection by their humanoid parents and peers. However, this is not always the case.

Unlike other brood parasites, if the Humanoid is already pregnant (but not showing), the changeling parasite does not eliminate the current child. They will develop as an identical twin (or triplets if twins are already present, although not necessarily identical if the twins are fraternal). This makes the Humanoid less likely to suspect and

The humanoid would not be selected by the changeling parent if the female is viably pregnant, this is because their egg needs time to gestate.