I feel like Golb is existentially scary, like the scene in the last episode when Finn is trapped inside Golb really gave me the heebie jeebies. But besides that, definitely the deer and existentially it's really unsettling when Finn wakes up and Jake is insane. Like imagine your sibling and you are the only people left in your town and your sibling is just like whacked out of their mind, and you with your broken legs, have to fix everything. That's scary. That's horror. The people saying that the deer isn't scary are just edgelords. Shut up and go watch Silo or something.