Demogorgon

The creature's mouth also enlarges and opens like a flower, revealing a row of sharp teeth. The skin is dark, the size of a dog, and the tail falls off.

Phase 1—Demogorgon larvae, resembling a common slug. They are small, usually dark green with yellow spots. The larvae inhabit the bodies of the Demogorgon's victims, including Will Byers and Barbara Holland.
Phases 2–5:
Phase 2 is a palm-sized, yellow-green demogorgon with a tail, arms, and a tiny mouth.
Phase 3 - The Demogorgon became dark green, gained a pair of legs, and thanks to its four limbs, became able to move quickly.
Phase 4 – The demogorgon grew to the size of a dog, its limbs became longer, and its skin darker. Its mouth became larger, opening like a flower, revealing a row of sharp teeth.
Phase 5 – The Demogorgon continued to grow and became the size of a Great Dane. It was able to dig a tunnel and escape the basement where it was locked, and it became much stronger. At this stage, the Demogorgon merged with other Demogorgons and became very angry.
Phase 6 - An adult demogorgon that stood on two legs, unlike the creatures in previous stages of development, and no longer had a tail
If the creatures are fed, they develop quickly and shed their skin when they change phases.