Biomes are defined by their plant life for a reason. Plants produce food from sunlight, everything else in the food web builds on that. Remove the plants and everything else dies. To have a true fungal biome, you need to be able to have the fungus feed itself, rather than devour dead/decaying/fecal matter from plants and animals (that in turn get their energy from plants). Fortunately for fantasy world builders, as every pixie taller than a mouse knows, fungi are the most efficient form of life when it comes to absorbing magical energies, even more so than the faerie races themselves.

As for a candy biome, that would be possible with sufficiently high tech, synthesizing sugar and other ingredients from non-living sources using (probably) solar power.

So, magic or magic-like technology can give you new environments.

"Horror" seems like just a modifier for some other environment. You can have a "horror swamp", "horror forest", "horror desert", etc. If you had an entirely undead ecosystem, feeding off magical energies (again), you might have something unique enough to call an "undead biome".