You might be interested in this thread I started a while back:
https://forum.inaturalist.org/t/whats-your-favorite-lost-species/30595
It discusses those “lost” species that may or may not still be out there somewhere. It’s pretty incredible how many there are.
They keep me awake too, but I unfortunately know I do not have the knowledge and resources to describe them.
I always knew there were undescribed species, but somehow thought they were “somewhere else” like the rainforest or the middle of Australia. Then I got interested in galls, and now I see a half dozen undescribed species every time I walk outside. Which makes me wonder how many other groups of organisms around me are also filled with undescribed species, and I just don’t know enough about them to tell.
As for more fun mysteries, my favorite has to be the reproduction cycle of eels. We’re finally beginning to understand it, which is really cool, but it’s been a mystery for thousands of years: https://www.smithsonianmag.com/science-nature/the-utterly-engrossing-search-for-the-origin-of-eels-180980777/
There is actually a theory that the mystery of the eel gonads is partially to blame for Sigmund Freud’s weird sex phobias.
Claus had the intention to prove that eels produce sexually, and set out to do this by locating a male eel. The year 1876 would find the young Freud assisting with this mission at Claus’ zoological research station in Trieste, a seaport city in northeastern Italy. Claus gave young Freud a rather monotonous task. For four weeks in what felt like an exercise in futility, Freud dissected hundreds of eels to try to locate testes. “All the eels I have cut open are of the tenderer sex,” he would report until around eel number 400 when finally, he found the prize: gonads, buried in the abdominal cavity. (source)
And then:
The only real surprise is how diffident Freud was about his discovery. Later in life, he avoided mentioning it – even going so far as to excise his report from lists of his publications. He may have done so simply out of disgust for the dissections he had performed. But another, more intriguing, explanation – hinted at by his friend, Ernest Jones – is that his frustration at being unable to find an eel’s testes may have triggered one of the sexual anxieties that were later to be so central to his psychoanalytical theories. If so, eels and their elusive gonads have a lot to answer for. (source)