Enemy Environment Reactions, sometimes abbreviated to EER, is a phenomenon that causes enemies to react to their environments in unique ways, varying on the environment and enemy. Some are cosmetic, others affect gameplay in some form.
| Environment | Enemy | Details |
|---|---|---|
| Ice/cold | Goomba | In icy or otherwise cold levels, Goombas seem to sometimes shiver from the cold. This is purely cosmetic. |
| Grass | Piranha Plant | In grassy levels, any Piranha Plants that are near or inside a patch of flowers will sometimes pet the flowers with their leaves. When sleeping, they try to use the flower patch as a pillow. |
| Cave | Multiple Enemies | In cave levels, some enemies wear mining helmets that makes them immune to normal jumps and punches. Ground pounds, sweep kicks, and any other type of attack, all work normally. |
| Haunted | Scuttlebug | In haunted levels, Scuttlebugs try to hang to the ceiling on a web, and then fall down in front of the player if they get too close, attempting to scare the player in the process. Their shadows are visible while hanging from the ceiling. |
| Fire/hot | Multiple Enemies | In hot or fire levels, most enemies sweat from the heat, with water droplet particles spawning and falling to represent the sweat. This is purely cosmetic. |
| Sand | Multiple Enemies | In levels with sand, most flower-themed enemies in the sand seem to wilt, making them move and/or attack slower than normal. |
| Water | Goomba | In levels with water, Goombas that enter the water stay in shallow areas, never going deep enough to drown. If they're chasing a player, they'll stop chasing them if they're too deep into the water. When they get out of the water, they dry off by shaking themselves for a few seconds. The player can jump on them while they're drying themselves off. |
| Fire/hot | Mr. I | In hot or fire levels, Mr. Is look like they're straining themselves to keep their eyes open, due to the heat. This is purely cosmetic. |
| Negative Aura | Skeeter | In levels with a negative aura, Skeeters will look absolutely miserable, their movements also getting more sluggish. |
| Ice/cold | Bully | Do I need to say it? |
| Negative Aura | Bully | Bullies get uncharacteristically violent, trying to outright headbonk the player, outright using their horns as a weapon, making them able to deal damage on contact. |