While I can't recommend which youtuber to I watch I can give you a tip!! I think you don't necessarily have to find a Japanese youtuber that speaks slowly since you can always adjust the playback speed to help get the words easily!! :)
I don't really watch anyone who only plays horror. I'll give you some of the bigger channels. Maybe you find something you like. I didn't pay too much attention to how fast they talk. I think half of them are kinda hyper and the other a bit more laid-back, idk.
I like horror so channels that mainly play horror games would be nice. Thanks!
AH I just found out about that yesterday , but it's on twitch ( you should be able to access without an account) ttakkunn - the name (one with the 35 followers ) whats also great is the English comments get translated in japanese for the streamer.
Small detail but rather than youtuber that speak slowly, try to find youtuber that articulate well. If they do articulate well and speak clearly, even if they're speaking fast, you will be surprised by how much you can understand. On the other hand, when they don't, even at slow speed, you may struggle. I watch a few clip like that, even with subtitles, where I couldn't hear the correct word. I rewatched the clip more than a dozen time, at normal speed and slow speed, but I still couldn't hear the correct word.
Other than that, I also quite like watching horror games (despite not playing them that much x) ). But I never really found one that specialized in horror game only. So instead, I usually type the name of a game with the name of a group of vtuber or youtuber/streamer. Especially when those game are trending, youtube will just feed you a lot of clips and streams. Even for past trending game like phasmophobia, lethal company, REPO, Mimesis more recently, etc. Those game are honnestly the best if you're starting to watch japanese contents. Because they are short and repetitive. And without a story. That last point is important because game with stories often used complicated, unusual or technical words even for japanese. Like for Resident Evil 2, I watch quite a few clips and streams where Japanese were unable to read correctly a technical word about biology. Or Silent hill f more recently that did use a lot of old japanese. Like 也 (なり). Even saw a clip where a vtuber read it as や and ask why there was some kansai dialect suddently (which was funny to watch but difficult to understand if you don't know what や means in kansai ben and you don't know that や is the cursive form of 也). So, if you want to enjoy it, I recommend starting with short repetitive game without a story for now (except if you're stuborn and not easily demotivated).
Oh and small tips, when you're searching for the name of a game, just search it in wikipedia and switch to japanese just to be sure (resident evil is バイオハザード for example).
ジャックオ蘭たん is a good one if you're looking for someone who does horror games. Although he does do a variety of genres, there's usually the occasional walkthrough of some indie horror game. He's also pretty laid back too.
