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