What is a long-term monster and what is its use for a story? Well, what I mean when I talk about a long-term monster is a creature which has been around for a long time and might hibernate for decades, just to pop up again for a snack or two or three before going back to bed. Stephen King’s Pennywise from the novel “IT” and the Creeper from the “Jeepers Creepers” movies are examples of that type. They stay dormant, then harvest fear and, in the Creeper’s case, new body parts, and return to their dormant state for long enough so the town forgets about them again.
The last is an important point. Both Pennywise and the Creeper rest for more than twenty years before waking again. Twenty-five years is the time span considered a ‘generation’ today - this means within the resting time of the monster, authority figures will change, people will grow up, move away, die of natural or other non-monster-related causes. Keeping the hibernation cycle close to that (23 years for the Creeper, 27 for Pennywise) means that the monster can work longer without being spotted as a pattern in the town history.
In “IT”, the protagonists have to face Pennywise twice - once in their childhood in which they manage to survive, but not to kill the monster, and once in their adulthood when it gets more challenging for them to oppose the monster, but they also manage to kill it.
This is one useful aspect of such a creature. You can have a prologue or an early part where the monster is around and a protagonist, then young and innocent, just gets away or loses important people in their life, and then you can have the second part where the monster comes back and the now-adult protagonist can take their revenge and kill it. Of fail in doing so, of course, as such creatures are most common in horror stories where the ‘happily ever after’ might not happen. With such an old creature, the bad end is more likely, given how long it has already been around.
From a storyteller’s perspective, another good part about such an old creature is that it can be followed through time. The protagonists or a side character can find traces of the creature in the town’s history - perhaps strange disappearances every X years or a mention of murders that are X years apart, but seem similar. There can be an old character who has gone through two or more periods of the creature’s hibernation cycle and remembers the last time people died that way or children disappeared or whatever you want to use for the creature’s signature move. Like this, the fear or suspicion of the protagonists can be confirmed - there is something horrid that kills every X years and it has been there for a long time. Their loved one from the prologue (or first part) wasn’t the first victim - and now they will stop this from happening ever again!
When you create such a monster, you need to keep in mind that it needs to have a consistent behaviour. Whenever it comes out of hibernation, it needs to have a clear MO for its hunting - always killing the same type of person (like a serial killer), using a signature way to kill them, kill the same number of people every time, kill during the same time span in days (as the Creeper’s 23 days), something like that. It must be possible to see that all of these deaths have something in common and that this something in the monster. It’s probably not bad to consider this type of monster a supernatural serial killer - one who kills only at certain points in time, such as every X years.
The monster must also pose a direct threat to the people who go up against it. Either they are hunted by the monster or someone they love has been killed by it or someone they love is currently targeted by it. They must be involved - this is not a topic for professional monster hunters (let’s be honest: they’d make it less exciting, having much better chances). This is where a prologue or fist part come in. You can work without that, too, though, if you make them the target and let them get away the first time they meet the monster (as the first “Jeepers Creepers” movie does).
These long-lived monsters are also hard to kill and it’s quite possible that they can go into hibernation and come back (this is suggested by the end of “Jeepers Creepers 2” where the ‘bat out of hell,’ an emaciated body, is still there 23 years later and it might be possible for the Creeper to come back when the time is right). For a horror story, though, an uncertain end, an end with a ‘?,’ can be perfect.
Useful backgrounds for such long-lived, hibernating creatures could be eldritch monsters, demons, alien creatures, or suchlike. They are not bound by the rules of human bodies (Pennywise is probably not even humanoid in its true form) or able to regenerate them (the Creeper harvests body parts to replace those which have been damaged or worn out). They do not follow a human definition of life. They might come from a completely different plane of existence or a world somewhere in the cosmos.
This fits together well with an aspect of horror stories that is often overlooked: a horror story is one of the few types of stories where a happy ending is not expected. In most types of stories, the protagonist should be victorious, get what they want, live happily ever after. In a horror story, the monster can prove too powerful for the protagonist and kill them or drive them insane or even take them over and use them against others. This then cements the power of the monster - and can suggest a sequel at some point, if you want to have one.
A long-term monster can be a good antagonist for a horror story because it pits the regular human with their short life span against something that has weathered the ages and seen many humans fall. It comes with extremely high stakes and makes for a high tension during the story. Yet, it might also require a prologue or a split of the story between two times. Make sure the monster has a signature way of killing and a clear MO so the protagonists can find out how long it has been in the area already and know when it is killing again. Then have fun with your eldritch abomination, demon, or alien or whatever you make the monster out to be.