Xenomorph skin is surprisingly slick for being a subterranean species. This is due to their bodies secreting a slick oily substance. Unfortunately, other species have discovered fire and how useful it is on oily things.

Xenomorph's are vulnerable to fire

An odd, but very useful, evolution Xenomorph's developed is a very strong muscle in their Oral Cavity. This muscle comes equipped with it's own set of teeth for gnawing and mashing. And it can be 'spat out' or ejected with deadly speed, instantly killing the victim and granting the xenomorph 5 temporary HP. Can be used only on grappled targets.

As a bonus action, you can attempt to use your Oral Muscle to penetrate the skull of your grappled target. They must have less than 10 HP and succeed in a dexterity saving throw to see if they can move their head faster. Dex saving throw is 8 + Your Dex Modifier + Proficiency modifier.

All Xenomorph's possess a natural, chitinous armor or Hard shell that continues to evolve as they age. 

Xeno Natural armor is 10 + Dex modifier + Proficiency Modifier

Not only do Xenomorph Adults have a deadly tail, the Drones also have a deadly bite attack with their razor sharp teeth! When making a Bite attack, make a melee attack roll and on successful hit the Bite does 1D6 + Strength modifier.

Warriors have evolved to grow large claws or talons on their hands, giving them another deadly weapon! When making a Claw attack, make a melee attack roll and on successful hit the claw does 1D6 + Strength Modifier.

When making an attack and are able to do multiple attacks per action, you can use any combo of Claws, Bite, and Tail Slash.

Due to the Millenia of fighting and evolving, all Xenomorphs have been sturdier and more robust with each generation!

+2 to Constitution, +1 to Dexterity

All Xeno egg's lay dormant once hatched and only open when they sense a suitable lifeform nearby. Once a victim is detected, the egg opens up and a Facehugger emerges, quickly moving to the victim and latching on. Once the facehugger is latched on, it forcibly impregnates the victim with a embryo and than dies. Over the course of a few hours the embryo morphs into a chest burster, where in it violently explodes from the chest of the victim, who usually does not survive long afterwards. The chest burster immediatly looks for a safe hiding spot where it molts and mutates into an Adult Xeno. Once the Xenomorph reaches adult hood, the imprinting from the queen takes place and the Adult becomes a drone to expand the hive, nurture the eggs, or dote on the Queen. Or the Adult becomes a warrior who specializes in quickly and ruthlessly dispatching prey to protect the hive. And on the rare(or very rare chances) the Adult becomes a Praetorian or Queen. The average life span of a Xenomorph is 100-120 years, but most do not live that long outside the exceptions of Praetorians and Queens.

All Xenomorphs are Medium creatures. 

Xenomorphs are naturally quadrapedal but can be Bipedal if needed. This allows them amazing movement speed over most other creatures, and allows for farther hunting. And due to their body's composition and flexibility, Xeno's can do many athletic feats. In addition, due to their history of violence, they are able to survive in most environments that is not a vacuum or on fire.

Xeno's have a base Walking of 40 feet, Climbing of 30 Feet, and Swimming of 15 feet. And are able to hold their breath for 5 minutes before needing to get more air.

Your tail is your most deadly weapon! As your Xeno get's stronger and evolves more, so too does the tail. Becoming sharp and deadlier!

Drone Tail at level 1 does 1d8 slashing with a range of 10 feet. At level 2 it does 1d8+Dex modifier slashing with a range of 10 feet.

Warrior tail at level 3 does 2d8 Dex Modifier slashing with a range of 10 feet. At level 6 4D8 + Dex modifier slashing with a range of 10 feet.

At level 12 8D8 + Dex Modifier slashing with a range of 10 feet. At level 15 16D8 + Dex Modifier slashing with a range of 10 feet. At level 20 32D8 + Dex Modifier slashing with a range of 10 feet

Acid is your life blood. It courses through your body as red blood courses through humans! This gives you a natural immunity to acid damage! This natural defense has two effects!  To remove the acid, Spend 1 action to wipe off the acid.

When you are dealt melee damage, the attacker takes 2d8 acid damage with a Dexterity save of 8 + Your dexterity modifier + your Proficiency modifier. On a failed save they take full damage and their body is covered in acid for One Minute, taking 1d8 acid damage on each of their turns for the duration. On a successful save, they take half damage and are not covered in acid.

When knocked unconcious, your body violently secretes your acid blood in explosion, covering everything within a 10 foot radius in acid. Acid Damage is 2d8 acid damage with a Dexterity save of 8 + Your dexterity modifier + your Proficiency modifier. On a failed save they take full damage and their body is covered in acid for One Minute, taking 1d8 acid damage on each of their turns for the duration. On a successful save, they take half damage and are not covered in acid. You can use this ability 1 + Proficiency Modifier per Long Rest.  At long rest all charges are restored.

Due to Xenomorphs being a mostly subterranean species, they do not have eyes. But inside their elongated head/brains is a electromagnetic sensory organ and vibration-sensitive tissue, granting them a form of echolocation. This grant's them blindsight up to 60 feet. And due to the vibration-sensitive organ,  all Xenos are able to understand languages spoken by other languages that emit noise or phoenitics.