In the film- Alien
, this was one of the manifestations of the killing machine
from outer space
. The creature, designed by H.R.Giger
, evolved during the course of the film. It's second incarnation
, after emerging from an egg sack, was a slimy, octopus looking creature that attaches to the face of a host animal (in this case a unwitting human). It wraps its tail around the throat of this person, preventing removal. The face hugger
has an acid based circulation, so when cut into spills a substance that eats through the steel floors of the spaceship
It uses the host animal for both a food source and as a breeding ground.
It reappears in the Alien sequels.
It's not pretty