Goomba - Super Mario Bros. series
Is weird because: It's a walking, semi-sentient mushroom. It has the ability to move using rudimentary limbs, and has a face. Real fungus does not possess any of these abilities or features. Also, it manages to insta-kill Mario on contact, seemingly without even biting him.
Could only exist if: The fungus wasn't working alone. The Goomba must logically be the product of some manner of parasitic fungus, like the Cordyceps strain seen in The Last of Us. The fungus attaches itself to an unfortunate host (say, a puppy or small child), grows through and around its body, and ultimately takes control of its basic brain functions, forcing it to walk endlessly back and forth in an attempt to find and spread to additional food sources. Hence the fact that Goombas are often found in groups. Its ability to kill on contact is presumably the result of a biological toxin released through the fungus' surface in order to protect it from being eaten by predators, or perhaps an ultra-fast acting digestive enzyme.
Given the state of the Mushroom Kingdom, it’s entirely possible that the region’s monicker is just a darkly ironic slang term, used to reference the area following a crippling countrywide outbreak.