Monsters by Design
Some monsters might have been created on purpose, from something already alive or something inanimate. The latter suggests great power behind its creation, such as a wizard or god, both found in fantasy but often not in SF. This suggests that fantasy can get away with monsters that are farther removed from humans and animals than science fiction, where mutation of pre-existing life is more likely. But there’s nothing that says technology, if it’s advanced enough, can’t create life from nothing.
Either way, our monster by design likely has a purpose. It’s possible someone was experimenting with creating life, like Dr. Frankenstein. But that’s been done, which is not to say that we can’t do it, too, but we should strive for a fresh angle if we do. Maybe the monster’s creation was successful and it’s here for a reason, such as guarding something. Or terrorizing villages at a wizard’s beckoning, which suggests story ideas. The creator should have some reason as to why they did it.
Maybe the monster is still fulfilling its purpose. Is it proud of that, or does it wish it were free of it? Is it bound somehow? Perhaps its purpose is long gone and it doesn’t know what to do with itself. Perhaps its master has been destroyed and the monster is on its own now. Or the master was imprisoned and the faithful monster intends to free him. The monster may never really have a purpose at all. These monsters might experience an existential crisis like Frankenstein’s monster, who is tormented by its existence and doesn’t understand what it did wrong to be cast out by its creator.
This sort of monster can also elicit sympathy if desired. Slavery is still slavery. The monster can be tormented by its existence. Maybe it will even welcome death at the hands of those seeking to slay it, but maybe it fears that others want to capture it, and so it must fight back. There’s always the monster that’s content with being one, which is probably more likely with a monster created on purpose than with a monster created by accident (and which may remember the past or be disabled in some way that makes its existence especially hateful).
Humans have a moral and ethical code. Monsters are depicted as not having one. This helps us use them as they were often intended, as a warning about untoward behavior. But monsters may have a moral code, too, just one very different from ours, where the killing of sentient life is no big deal to them, for example. Their creator might’ve wanted to imbue them with certain traits, like protection of them or an item. That raises the question of how much control the creator had/has over the monster. Is the creator dissatisfied with his work? Are monster and master bound in mutual loathing but dependency? If the master is killed, will the monster die, and vice versa?
Who Caused It?
Who would create a monster on purpose? Wizards, gods, and “mad” scientists come to mind (like Dr. Frankenstein and Dr. Jekyll). So do bad people who’ve captured others and who want to see what happens when someone is exposed to something dangerous. It’s possible that no one knows who created a given monster, meaning we don’t have to mention it, but stories do well when there’s some speculation about a monster’s origin. Mystery is good, but not total mystery, which is why we all love clues.
Any character willingly creating monsters is likely up to no good. The creator’s motivation should be understandable even if the logic behind what they’ve done is terrible and twisted. In fantasy, gods are a great choice because they might like creating beings for reasons never explained to the species. Or maybe they just want to keep life interesting for sentient inhabitants by creating monsters they have to deal with. SF offers many ways to expose someone to phenomenon, including viral infections from new worlds.