Trilobite (Prometheus)

The Trilobite is a creature derived from the Black Liquid of the Engineers.

History
The liquid, when ingested by the human Dr. Holloway, spreads through his system within a 24 hour period. Before realizing the changes taking place in his body he and Dr. Shaw had intercourse whereby transmitting some of the changed DNA to her in the form of sperm. She soon became pregnant. However, she had the rapidly growing alien inside her surgically removed via cesarean using an advanced medical device on board the Prometheus. The creature, presumably dead, appears again towards the end of the movie resembling a massive Facehugger which grapples with and impregnates the Engineer with a Xenomorph-like species, the Deacon (termed "Proto Alien" by several online references).