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