As an average Xenomorph, your height is between 7-8 feet, and your weight is between 136kg to 182kg. Your size is medium

Your constitution score increases by 2, and your dexterity score increases by 1.

Xenomorphs mature extremely quickly, maturing from eggs into fully formed adults in a matter of months. The Xenomorph can live up to 100 years if not killed.

The Xenomorph has a base walking speed of 30ft, as well as possessing an innate climbing speed of 30ft.

The Xenomorphs do not possess eyes, and as such do not see in the traditional sense. They can get by as well as other creatures can using a combination of echolocation, as well as the unique ability to read electrical impulses in order to sense their surroundings. Blindsight of 30ft. 

• You gain a immunity to acid damage.
• If an enemy hits you with a melee weapon or unarmed attack that would break skin, they take 1d6(1), 1d8(6), 1d10(11), 1d12(16) points of acid damage.
• When you fall unconscious, you release a blast of acid hitting all creatures within 5ft of you for 3d6(1), 4d6(6) 5d6(11) 6d6(16) acid damage. The enemy can make a Dexterity saving throw in order to half the damage. The DC for the save is equal to 8 + your proficiency modifier + your constitution modifier. You can use this ability once per long rest.

You only know xenomorph as a base language but understand body language and gestures as broken common and learn new languages at an accelerated pace by just listening. You can never speak in any language but xenomorph, but can learn to read write and understand in all languages over time. 

Your exoskeleton, though it is tough, it is not invulnerable, you are vulnerable to fire damage. In addition you have disadvantage on saving throws against extreme heat.

Bite. You have a bite attack. If you hit with it, it deals piercing damage equal to 1d6 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.

Claws. You have a claw attack. If you hit with it, it deals slashing damage equal to 1d6 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.

Tail. You have a tail attack that has a reach of 10 feet. If you hit with it, it deals piercing damage equal to 1d4 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.

This effect increases in damage by 1d6/1d4 when you reach 6th level(2d6/2d4), 11th level(3d6/3d4), and 16th level(4d6/4d4).

The alien undergoes a mutation, changing its body in a new way that makes it different than standard aliens. The alien chooses one of the below abilities, which then cannot be changed. At 16th level, the alien gains an additional mutation of her choice from below.

Extending Tail : The praetorian's prehensile tail is elastic and coiled between the exoskeleton plates, allowing it to stretch outwards to great length when attacking, but it also allows the tail to be used when performing the following maneuvers: grapple, trip, and simple interactions with objects. Its tail attacks are now considered primary attacks with 15 ft. reach.

Pounce :The Alien gains the pounce special ability. When a alien with this special attack makes a charge, it can make a leaping attack from without run up, up to 15 feet away (double if at least 20 feet of run up). If the attack connects deal 1d8 extra bludgeoning dmg and knock target prone.

Aqualung (Ex): The Alien's torso expands slightly, its body growing a third lung capable of processing water into breathable air indefinitely.

Flight (Ex): the alien grows leathery bat-like wings from her shoulders, gaining a fly speed equal to its base speed.

Reaver Blades (Ex): An Alien grows large blades from the wrists that can be used instead of its claw attacks, dealing far more damage than normal and being treated as +1 magic weapons for overcoming DR  2d6 slashing plus 1d6 acid

The Xenomorph molts for the first time, becoming Praetorian. The Alien’s size increases by one step, increasing Strength and Constitution by +2 and decreasing Dexterity by -2. The Alien gains +3 natural armor. Your size also changes to large

The Praetorian molts for the final time. The Alien’s size increases by one step again; increasing Strength, Constitution, and Intelligence by +4 while decreasing Dexterity by -2. The Alien’s natural armor increases to +6. Your size is now huge.