Specimen 6 is a Xenomorph
Specimen 6 began his life as a small embryo implanted into a Human host. Once matured he bursted out of his host's chest. Little did he know he was being examined by Weyland-yutani scientists and that he was trapped in a tube and when evolved was then put into restraints, the scientists branded the number 6 on his head And began their testing.
During testing a power shortage accured and this was the perfect chance to escape ,during the confution