Xenomorphs.
Let's start off with the little eggs, shall we?
The egg doesn't seem dangerous. But when it hatches, and you peek inside just like any curious human being would do to know what's inside. A Facehugger then leaps out of the egg to attach itself to your face.
Then the Facehugger impregnates it's host by putting an embryo by the host's mouth.
Then after a period of gestation, the embryo evolves into a chestbuster which erupts from the host's chest and killing the host in the process.
The Chestbuster then evolves into an adult form and because of the gestation period inside the host's chest, the now Xenomorph takes some physical attributes of the host. Making it adapt to the same environment as the host.
Now the Adult Xenomorphs... They are capable of running and crawling along ceilings and walls, possesses great physical strength, have blade tipped tails, have extremely potent acid as blood and have a retractable tongue which got it's own set of snapping metallic teeth that can easily break through skin and bones.
If this doesn't give you nightmares, then I don't know what does.