Changeling

Changelings are humanoid creatures that have discolored, slimy and wet skin, hollow and sometimes no eyes, and a wide bending mouth with many sharp teeth. They can appear human, but their true nature is revealed in reflection. They feed on humans, specifically on synovial fluid, and the essence of the soul. Lucifer mentions them to Sam.

Characteristics
They have been talked about throughout history as being replacements for human children who are stolen away by monsters. The children are usually taken underground, and fed on by the mother changeling. The changeling that replaces the child looks the same as the original child, but is evil, and otherwise acts differently. They feed off the mother of the child they have taken the form of. They are very protective of their food source and will let nothing separate them from it and will kill anyone who gets in their way. The mothers get fed on gradually and will last a few months before death, at which point the changelings move on to the next neighborhood.

Although they act somewhat like children, some mothers are able to notice that it is not their child. This is due to the mechanical way that changelings act, repeated spouting generic "childish" phrases, at times with little to no emotion behind them. Likewise, changelings have the habit of staring at people "like [they]'re lunch."

Physical Appearance
Changeling's true form is discolored, slimy and wet skin that appears to be molting or covered with varicosities, hollow eyes, and a wide, gaping mouth with many teeth. A changeling's true form can be seen only in a reflection in a mirror or other reflective surface.

Physical Abilities

 * Super strength - They have superior strength to humans, however, they are quite weak compared to most monsters. Young changeling is not strong enough to break through a door.
 * Lamprey-like mouth - They have sharp teeth that allow them to feed on humans, and a lamprey-like mouth, which leaves a mark on the back of the neck.
 * Shapeshifting - They can appear human, with their true appearance occasionally appearing in their reflection.
 * Super agility - Are able to climb walls to snatch children.
 * Invulnerability - Can only be killed by fire, but are not completely invulnerable, and can still be apparently stunned by blunt force.
 * Teleportation - One changeling demonstrated the ability to get out of a car that had sunk into a lake and appear back at "her mother's" house within minutes, soaking wet and unharmed.

Weaknesses

 * Fire - Setting a changeling on fire will cause its body to rapidly burn to ashes.
 * Family - Killing a mother changeling will cause its children to die instantly.
 * Mirrors - A changeling's true form is revealed in a mirror.