In speculative fiction — and among a variety of interest groups on YouTube — organisms unknown to muggle science proliferate. Dragons, elves, Big Foots, and Klingons are taken as given, since one of the main appeals of the genre is an exploration of worlds that are not ours. For what are called soft magic or science fiction systems, this works. The characters or the adventure occupy our attention enough that we are not troubled by questions from ecology about how such things can work. But if realism is a concern, things are not so simple.
This is addressed in George R.R. Martin’s A Song of Ice and Fire series, despite his focus being more on characters and on the grey nature — between black and white — of human action. Early on, Daenerys acquires and hatches dragon eggs, which plants the story in fantasy, though raising the worms is atypical. But in later books as Daenerys grows into political power, the reality of having three carnivorous and growing beasties is brought to the readers as her subjects come to her to demand compensation for lost sheep and children. The people featured in the Tiger King travesty should take note.
This bureaucratic reality — powerful weapons require a supporting infrastructure and will cost money — is a valuable corrective to the presence of whimsical beings with no consequences. I do not mean to be a killjoy. A fairy tale needs less in terms of logistics, and The Flintstones can get by on cartoon rules. But Martin is striving for a different level of fiction. And his realism is one that would benefit more than his fellow writers. The number of cryptozoologists and producers for the SyFy Channel should consider whether Mokèlé-mbèmbé, Nessie, or Big Foot have enough territory remaining to sustain themselves. And the carrying capacity of a region is only one question. As rarely as these individuals are sighted — presuming that they actually exist, of course — they appear to be at a population bottleneck in which the total of breeding adults is insufficient to provide the genetic diversity necessary to maintain the species.
The Witcher series deals with these problems by showing communities such as elves that have been forced by humans away from arable land into the mountains and are suffering a decline as a result. A similar solution is found in J.R.R. Tolkien’s works. The elves in The Lord of the Rings are on their way out, and while this is the result of millennia of conflict internally and with a couple of dark lords, their decline is also due to the rise of humans as the winners of the existential crisis. Lórien, the realm of Galadriel, is the shrunken remnant of former vast territories, both in land and in its name, along with a few other enclaves, and the elves are migrating westward, having realized that they can no longer keep the competition out of their resources. Why this is the case may be the result of the lifespan of elves. Human live many fewer years and so have a greater urgency to reproduce in quantity — a problem that elephants experience, at least in so far as taking a long time with each child.
If your writing is going to be realistic in addition to fantastical, these factors have to be taken into account to have sustainable populations in your stories. How does a particular species survive — what are the resources that they use, what are the ecological challenges, and how are they addressing them — is a question that will suggest plots and characterizations that go beyond a quest to move the MacGuffin across the game board.