A world with monsters is arguably more entertaining than one without.
Appendix 4 is a template for creating one. It includes more comments and advice, and an editable Microsoft Word file can be downloaded for free by signing up for the newsletter at http://www.artofworldbuilding.com/newsletter/
We all know what a monster is, but since we might be creating species and animals, too, let’s be clear. The term implies something harmful, unnatural, and morally objectionable, whether there’s a physical deformity or psychological one. Monsters aren’t real, of course, and are created by storytellers, usually to depict or highlight some of the above, sometimes as a warning. They are often a freak of nature and can result from birth defects, in which case it was something else, like a human, before being regarded as a monster by horrified onlookers. Their existence has often been thought to foreshadow something evil happening, which is one reason they are cast out.
In science fiction and fantasy, the word “sentient” is used to describe creatures that are human-like in their mental capabilities, even though that’s not what the word really means. Due to this convention, this usage will be retained herein anyway. The real definition of sentient only includes the ability to sense, feel, and experience, which means an animal is technically sentient.
As a side note, with space traveling characters visiting new planets, what they might term a monster at first might turn out to be an indigenous animal. Either that, or it’s a member of a species that might’ve been stranded, for example, and terrifies those near through no fault of its own except appearance, and it’s assumed to be a monster.
Monsters vs. Species
The difference between a monster and an intelligent species is arguably their minds. A humanoid species is typically sophisticated in having what humans have: society, culture, philosophy, and other aspects that distinguish us from animals. This is a generalization, but monsters don’t typically have these things, or at least, not in a way beyond that of animals. We can argue, rightly, that animals like dolphins and apes have a certain social structure, but these are communicated as much with body language as verbally. Any language is fairly limited compared to mankind. They don’t read and write or pass down long histories. A generation today likely has no idea what was happening one hundred years ago, though this is admittedly conjecture.
None of this means we can’t have an intelligent monster if we choose to, but once we start giving a monster these things, it starts moving in the direction of humanity. We may find ourselves deciding that our monster is very cool and could be more useful, so we turn it into a species. No harm in that. Dracula is a good example of a smarter monster, but while he is a vampire now, he was once human. This is also true of zombies, who are typically portrayed as relatively stupid. We can use a similar approach (the monster was once human) to explain our monster’s sophistication.
Does monster automatically mean unsophisticated like an animal? In fantasy, SF, and gaming, yes. They’re typically portrayed as things that can’t be reasoned with when one corners us for dinner or we wander into its territory. In this sense, they’re just like animals. We likely can’t communicate verbally with it, either, but that’s not a rule either. We can teach pets to understand what words mean, but that involves frequent time together, a reality that would make someone no longer think of the monster as such, most likely. This raises the idea of most people thinking it’s a monster but one person having befriended it, which has been done with children’s stories.
Monsters vs. Animals
A major difference between monsters and animals is numbers. Just about every monster we’ve heard of was a “one off,” meaning only one existed. The reason is the aforementioned purpose in a story—to teach a specific lesson that didn’t require more of them—and because they are abnormal, which by definition means uncommon.
This is not to say we can’t have more than one. Zombies and vampires are good examples, but in both cases, these originated from humans and we don’t generally consider them monsters even if they’re monstrous. In a film series like Aliens, we have what appears to be a monster but which is really an animal. Why? Numbers.
If we have more than one, can we still call it a monster? Sure, though “creature” might be a better term, but if we read the definition of that, it means animal. Does our audience care about definitions? Probably not. Either way, the existence of two or more identical monsters benefits from a good explanation, such as both of them being created at the same time as in a laboratory accident. Even then, there’s no reason to say that the accident caused identical mutations. One accident could cause twenty different monsters, not twenty of one type, which are capable of reproducing and then being considered animals. Once we start multiplying them, we’re going to start needing a name for them, at least, and unless they scatter, they might start developing more sentience (like society and language), sooner or later, and start becoming a species.
We needn’t ever refer to our creation as a monster, but people use this term partly because there is only one. It has no name. It’s an “it,” as well, not a he or she, even if it appears to have a known gender. No one knows what to call it, unless it’s been nearby a long time and someone gave it a nickname that stuck. In the book Frankenstein, the monster has no name, but because promo efforts for the movies use the monster prominently, people often think its name is Frankenstein when that’s really the doctor’s name.