Praetorian

From AvP Online Encyclopedia

Praetorians are large, female Xenomorphs featuring many resemblances to the alien Queen, but smaller. The Praetorian is known for its harder-than-normal exoskeleton, and it's job as a protector to the royal class of alien (usually Queens). Praetorians are sometimes refered to as "young queens" or "royal guards". When the Queen of a hive is killed, a Praetorian is able to take her place by molting into a Queen.