A mysterious deep-sea creature that no human has EVER seen...

 https://www.dailymail.co.uk/sciencetech/article-12490773/Mysterious-golden-egg-discovered-seafloor-Alaska-belong-deep-sea-creature-unknown-science-experts-claim.html

If there was ancient life on Mars, perhaps some of it was also on Earth.

https://www.smithsonianmag.com/air-space-magazine/life-may-have-been-more-likely-originate-mars-earth-180978021/

https://www.psu.edu/news/research/story/traces-ancient-ocean-discovered-mars/

https://www.dailymail.co.uk/sciencetech/article-12490957/NASAs-Perseverance-rover-spots-shark-fin-crab-claw-Mars.html

https://en.wikipedia.org/wiki/Panspermia#Interstellar_or_interplanetary

https://en.wikipedia.org/wiki/Directed_panspermia

In a way, the movie Life_(2017_film) is like a contemporary story about intelligent organic matter from Mars migrating to Earth. https://en.wikipedia.org/wiki/Life_(2017_film)#Plot

Of course The_Thing_(1982_film) is also a story about a hostile ET life form crashing on earth, but a very long time ago.

Horror_Express (1972) is another fascinating agressive ET on Earth story.

"The extraterrestrial has been stranded on Earth for millions of years. It kills passengers with specific knowledge that could help it build a new spaceship. Eventually, Cossack Captain Kazan stabs and shoots Mirov. With Mirov dying, Pujardov, believing the creature to be Satan and having pledged allegiance to it prior, allows it to possess him." https://en.wikipedia.org/wiki/Horror_Express#Plot

Quatermass_and_the_Pit_(film) is an interesting story. https://en.wikipedia.org/wiki/Quatermass_and_the_Pit_(film)#Plot

https://www.tcm.com/tcmdb/title/24796/five-million-years-to-earth/#overview