The Deacon was an artificial creature that was created from the Trilobite's embryo that was impregnated into an Engineer. It later burst out of the dying Engineer's chest after David and Elizabeth Shaw left LV-223 for the Engineer homeworld.
Due to its strong resemblance to the Xenomorph, many fans have speculated that the Deacon might be either an Engineer-spawned Xenomorph, or the progenitor of the Xenomorph species.
The Deacon bears a strong resemblance to a juvenile Runner, as well as sharing the same method of exiting its host's body.
Deacon itself means "minister", "waiting-man", or "messenger"
Due to Xenomorphs often being larger then their host species, judging from the height of the engineer, the deacon may grow to huge sizes.
The Deacon does not have an "attack tongue" as most other xenomorphs have, but instead has a second jaw in the inside of its mouth.
Seeing as how Xenomorphs take on some of the DNA of the host, the Deacon could have more strength than regular human-xenomorphs.
If this is the first alien, this may be a praetorian, and other humans may have visited this planet later on, seeing as how it got from LV-223 to Hadley's Hope.