Bioshock 2's Big Sister was originally intended to be a constant threat, which would be hanging around Rapture waiting for you to hunt you down. Despite the fact that in its final version it was not entirely like that, it did make the player feel watched, with small appearances or shadows. It's supposed to work, but there are no times when Big Sister finds you unpredictably. Prince of Persia: Warrior Within had something similar to the Dahaka, or also the transcript of the Dark Side of the first Kingdom Hearts game, which every time it appeared was like playing cat and mouse, but as much as some games insist In which we are in a constant flight, there was an evident reality, and that is that most of them are programmed to be just a scare, more than anything not to be a particularly cruel game.
In video games, it is very difficult to find a real threat that chases you incessantly. Taking away the status it does have in the genre, Pyramid Head's appearances in Silent Hill 2 are very few, and usually tend to be in very tight spaces. Outlast's most fearsome enemies can track you if they know you're there, but if you leave their area they will stop chasing you. You could say that an enemy that does a little bit for this purpose is Nemesis, the main face of Resident Evil 3, but it is a somewhat limited enemy. In fact their confrontations and encounters are usually predictable, and of course there are ways to deal with it.
It is ironic, because this is an enemy that is limited on certain occasions of the game, which took the tension off the game to give the player a break. Although this is a fact with all the enemies of this style, and as such, it will come and go according to the situations of the game, but there is also a great truth, when the xenomorph is loose and is free to do what he wants, he is unique.
It is not that the creature has incredible artificial intelligence, because in fact others have done the same and better, but what is really relevant is that it is a threat that constantly haunts you on an open map. Something that makes this enemy different from other similar ones is that it breaks the barriers that limit this style of enemy, it can go wherever and whenever it wants, it is unpredictable. When they tell you that he is moving through the ventilation ducts, it is serious. You can be doing anything, even saving a game, but if he wants to appear at that precise moment, it will be.
If you have already played other games in the survival horror genre, you will be used to the fact that there are usually enemies. This is part of the progression of this style of games, because with it you can teach the player to use the mechanics of the game or scare him away, to make him think about whether to spend all the ammunition he has or run away. Finding a particularly strong enemy in any game of this style can be a barrier, but very few times will you find one that changes the rhythm of the game so radically. When the xenomorph appears, it changes the rules of the game, it is much faster and stronger than you, and due to its programming, it can get bored and leave the room it is in, and then return to it, it is unpredictable. It is not a fight to try to kill him, but to flee from him, he is an enemy who, if you do not have the necessary tools, can kill you with two blows, or sometimes directly kill you with one. In other words, he is the hunter, while you reduce yourself to being the prey.
But Nemesis was also very strong and was constantly following you. What differentiates the xenomorph from this enemy is that, despite the fact that the story is programmed so that it does not appear suddenly, it can appear if you attract a lot of attention. And attracting attention may be running away or making a lot of noise, which can attract the creature. Change the player's attitude, with the thought that it could appear at any moment, even at the most unexpected moment, even if you are fighting with some other enemy such as humans or androids, you can still do it.
I can't think of another video game that uses this mechanic in such an interesting way, and that really feels like a threat and instills terror, with a really free creature, a full-blown predator. A persistent entity that won't leave even if you change zones. Resident Evil 7 uses a similar mechanic, with Jack Baker, who will chase you and leave sometimes, but his presence is limited to specific areas. Alien Isolation isn't exactly scary, but he knows how to create panic situations that weren't planned to happen, he's arguably the biggest representative of the Alien franchise in video games.
