Horror biome elements:
Plants that should be dead, but keep on living or at least not properly dying. Trees without leaves that keep slowly growing. Trees covered in masses of fungus, mistletoe, and other growths that are either parasitic or feeders on necrotic wood, but which go on as if nothing were odd. Withered weeds that just stay as withered weeds for years and years. Flowers that dry up, but refuse to drop a single petal.

Plants where the flowers consume what would be their normal pollinators.

An abundance of parasitic life forms.

A lack of decomposers, leaving the dead to not rot quickly and be done, but to lie slowly eroded by inorganic forces.

Eyes, mouths, and other body parts in things that should not have them: Trees, rocks, mud, the doors of houses, fog...

Oh, more fog than the climate should support.

Blood from unnatural sources. Creatures that appear normal, but when cut or butchered, they have no blood.

Undead food chains. So many undead that the only way to sustain the populations is to have undead that feed off the energy of other undead.

Dogs that talk, but with a comedic speech impediment.

Way more tentacles than bestowed along our Earthly evolutionary path.

Creatures with transparent skin.

Human artifacts decay to the point of creepy unpleasantness, but no further. That house of rotting wood and sagging roof and crumbling brickwork? It looked pretty much the same eighty years ago and survived three hurricanes since. Doesn't mean a stair might not give under you, but it isn't going to fall apart if it isn't dramatic and dangerous.

Seeds grow wildly different plants than the ones that produced them. Animals and people give birth to completely different species.