If you are stuck on what to do for retro, here's some examples

3D styles

PS1 style: just go to the itch game page, you'll find like 300

N64 style: good examples consist of Catastrophe Crow, Shipwreck 64 and Kitty Kart 64

 Old 3D Computer games: examples are stuff like Baldi's Basics

2D styles

GameBoy or NES: the GB game jam wrapped up not to long ago so if you want to see a bunch of examples you can go here https://itch.io/jam/gbjam-12/entries

Old 2D computer games: point and click style games or educational style games, a good example is Mr. Tomato and Ms. Lemons which are horror games based around educational games

Arcade: not fully a style and more of a gameplay thing but this can also work, a good example is something like Pony Island

Other styles

Old timey cartoons can work, examples like Bendy and the Ink Machine or (while not a horror game) Maus which is a newer game

Other consoles: you can get more specific with it if you want, think of stuff like the virtual boy, Atari or the Sega consoles

Gameplay Based

The game's art doesn't need to be retro you can be creative with it. Some games use retro tech for their gameplay

examples include: Amanda the Adventurer, Through the Static, Analog horror can also work.

Some notes

if your game is based around an older console like the game boy or Atari, try and match the number of controls they have. You don't exactly have to follow this but retroness isn't just how it looks.

For retro tech, keep them to media based tech, an old phone will not count but old TV's and VHS will count as they add something visual for retro. 

Hope this helps!