The Xenomorphs of the Alien series were designed to be as alien – and dangerous – as possible given the budget and technological limitations of movie-making at the time. They have bladed tails; extra jaws; acidic blood and spit.
What I’m wondering is what kind of creature, presumably an imaginary one, would find a normal human being as physically dangerous as a Xenomorph is to us, putting aside humanity’s accomplishments as tool-makers and strategic thinkers. I’m thinking more along the lines of what it would take to devise a being that would be scalded/melted by human saliva or blood. What sort of bodily chemistry would such a creature have? It would be rather interesting to read a scifi novel in which a rogue human runs amok on an alien spacecraft, furiously slaughtering the poor aliens left and right using only his or her spit, blood, or nails.
Well, human blood actually is corrosive and does attack things it gets on. This is a big problem for developers of medical implants. However, it is very slow at doing this, obviously, and the cells in human blood that do this will obviously stop functioning shortly after the blood leaves the body.
I can imagine some kind of extremely light-weight alien creature that evolved on a much lower gravity world. Imagine Europa if it were closer to the sun. An ordinary human would be a rampaging monster with super strength compared to a being that evolved in 1/6 gravity or something.
The problem is, such a planet would have a much lower atmospheric pressure. Humans running around inside their ship would suffocate. (versus in Aliens, where it did not appear that low pressure and other environmental damage was all that harmful to the aliens, like when the queen stowed away on the outside of the shuttle)
The Henders Island creatures from Warren Fahy’s excellent Fragmentare remarkably allergic to salt water (which is the only thing keeping them on their island). As such, they actively stay away from sweating humans.
Saliva contains enzymes such as amylase, which breaks down starch into sugars, and lipase, which breaks down fats, as well as various antibacterial compounds. It’s conceivable that an alien life form could have an integument that was particularly vulnerable to such chemicals, although the reactions might not be that rapid.
Plenty of organisms breathe through their skin or other surfaces - flatworms and nudibranchs come to mind. To these organisms, exposure to human saliva or blood on the respiratory surfaces would be dangerous.
The catch is that most animals which breathe through surfaces on the outside of their bodies are rather small… plants and fungi get rather larger, but might not have the metabolic pace to be a good comparison to the human/xenomorph analogy.