Source: Star Trek (Original Series)
Threat Assessment: 3. It can appear in different forms to different people, and it can suck all the salt (and life) from a victim with a touch. It's strong enough to take a punch from a Vulcan.
Limitations: The last of its kind. Not ambitious, all it really wants is to be liked and suck salt from people which is admittedly conflicting goals.
Personal note: I remember this scaring me as a kid.