Campestri are... well, alright, they're not the silliest monster in Dungeons & Dragons, but they'll probably make most peoples' Top 20 Goofiest list. Distant and primitive relatives of the Myconids, Campestri resemble mushrooms some 3-18 inches tall with human-like faces and the ability to move by squidging along on their rootball. They vary widely in color, from white to tan to dark brown, but they always have red or purple caps and speckles. First appearing in Dungeon Magazine #41's "Old Man Katan and the Incredible, Edible Mushroom Band," they subsequently made their way into the Monstrous Compendium Annual 1.
Campestri live in "herds" of some 4d6 mushrooms, eating salty soil and slowly purifying it of its overabundance of salt. This, incidentally, means that they are extremely salty themselves - at least as much as caviar, apparently. Most creatures won't eat them, as a result, although bullywugs think they're a delicacy.
Individually, campestri are only slightly brighter than the average household cat, with an Int score of 2-4. But, in a group, they can pool their meager brains together and get somewhat smarter, getting closer to a 6 or 7 on the Intelligence scale. Dumb, they might be, but campestri aren't malicious - indeed, they're very happy-go-lucky creatures with few cares or worries, a warped sense of humor, and some rudimentary powers of reason. They're pretty harmless; they can "bonk" somebody with their head for 1 point of damage, but their main defense is to use their ability to unleash a 10ft burst of slowing spores once per day and then bounce off for their lives.
Technically, one could call the campestri a species of Chaotic Stupid mushroom-men, but really that implies a level of malice that they just don't have. They're simply... not very smart. This is highlighted by their introductory adventure; whilst Old Man Katan thinks they're pests, the truth is that they are not only very fond of him, but their caterwauling during the day is their way of warning him to the presence of giant mosquitoes - his reaction of lighting up one of his noxious homemade cigars in frustration has actually been saving his life, though he hasn't figured it out before the PCs arrive.
Perhaps their most notable trait is that campestris are captivated by any sort of singing, even incredibly bad singing. If anyone sings or plays an instrument, the campestris will happily sing along. The mushrooms can easily imitate both words and music. Once they have run through a song or piece of music three or four times, they remember it, although they have a tendency to mix and match parts of different tunes. The campestris will dance all around whoever is singing to them, enjoying themselves immensely.
Their AD&D statblock actually includes directives for DMs to encourage players to put up with their goofy efforts at singing by promising XP for anyone willing to actually train the things, with a score of 150 to 300 XP depending on just how good a job the DM thinks the players have done at playing along.