Start with a story that you want to tell first and foremost. What elements does the horror emanate from? From a story perspective, this fetish can shoulder horror elements quite well: force feeding; hard vore; gore and the grotesque; monstrous transformations; the manipulation, subversion and loss of will and control. And this is on top of all the more standard tropes that prey on player fears.
Horror games play on three key, persistent elements: 1) Ratcheting Tension, 2) The Unexpected & 3) Loss of Control. All monsters, blood and general freakery is just window dressing for your story but if you want the player to actually feel horror then you will dip into these three. How you interpret and utilise them is down to you; there’s no hard or fast answers here as the use of these will entirely depend on the story.
Do you want the player to be a psycho feeder that toys with their victims, stuffing them cruelly in bizarre experiments? Of flip the scenario, what about a terrified captive trying to survive a twisted situation or trying to save a helpless captive strapped to some monstrous feeding machine? A normal scenario that takes a creeping & imperceptible descent, or a sudden & violent lurch, into the bizarre? Big grotesque spectacles or small insidious details that fester in the player’s mind well after playing.
Erotica and horror are bedfellows at the end of the day based on how both prey on similar stimulations, however it is made trickier by the virtues of both being a game (where players having so much agency or repeated play can rob tension out of a scene) and one that caters to a niche erotic interest - done poorly and you’ll have a game that’s neither sexy nor horrific. This is why it is important to plan out a good story or premise.
It needn’t be a full narrative. For a quick, simple game you may just need an appropriately weighted premise. There was ages back on a site called Stuffedonline a simple Hangman game with the situation being that of a cute anthro lab mouse being forcibly fed whilst wearing a Saw-esque jaw contraption as part of an experiment. Each wrong answer led to her consuming ever more pies and the situation becoming ever more distressing for the mouse (and maybe even the player, haha).