A monster originating out of the Fiend Folio for Dungeons & Dragons 1st edition, then reappearing in Advanced Dungeons & Dragons' Monstrous Compendium Annual II and the "Ecology of the Flumph" in Dragon Magazine #246, Flumphs are one of the most divisive of all D&D creatures due to their inherently ridiculous nature, a category they tentatively share with the likes of vegepygmies, froghemoths, owlbears and gelatinous cubes.
A flumph is a small aberration that looks something like a flying jellyfish... only with even less dignity. Essentially, a flumph is a vaguely pancake-shaped mass of flesh with myriad shapeless tentacles mixed in with a half-dozen or so long spikes on its underside, a hidden orifice that "expels compressed air" to move through the air as it levitates - its name actually comes from the "flumph" noise it makes when doing so - multiple orifices that can spit stinky slime, and two bobbing eyestalks. If you're wondering what the spikes are for, they're essentially spider-style fangs; the flumph feeds by dropping onto small creatures (like rats, giant bugs, goblins and kobolds) and impaling them on the spikes, which secrete powerful acidic venom and digestive juices into the victim. When they're dead, the flumph sucks up the liquified innards through the spikes.
Now, this would just be any other stupid pseudo-alien critter, except for two things. Firstly, flumphs are smart - smarter than most people, in fact. Secondly, flumphs are Lawful Freaking Good aligned. In fact, some say they were the only Lawful Good creature in the entire Fiend Folio! So, yeah, this bizarre, alien critter is actually on your side, if you're not playing a villainous party.
Unfortunately for the flumphs, this same dichotomy ended up making them one of the most generally hated and despised creatures in the Monstrous Manuals. Although they have hardcore defenders, flumphs were, for a long time, dismissed as ugly, stupid, inappropriate, boring and generally one of the worst ideas to come out of D&D. To put this in perspective, in 3rd edition, flumphs didn't appear in any of the five Monster Manuals, nor did they appear in the Lords of Madness sourcebook, which was all about aberrations. Their only official 3.5 stats were in Dungeon Magazine #118, and the third-party bestiary "Tome of Horrors". Likewise, they were left out of 4e entirely, save for an April Fool's themed adventure released online, perhaps in part because 4e was quite firm on the design mandate that "if it's Good aligned, it shouldn't really be in the Monster Manual; those books should be full of creatures you can realistically expect to battle", which is why Metallic Dragons were reinvented as Unaligned for that edition.
But, in recent years, flumphs have seen something of a turning point. Pathfinder may have started it, with its "Misfit Monsters Redeemed" splat, where it answered the question "why are Flumphs Lawful Good" by making them a race of world-travelling crusaders against evil aberrations and cosmic horrors. Yeah, that's right, the goofy space-jellyfish is basically a star-faring Paladin that wants to defeat Cthulhu, and just has to recruit adventurers for the job because it's still... well, hilariously weak by comparison. Meanwhile, 5th edition D&D actually brought them into its Monster Manual, the first time that's ever happened, and claimed that they are a peaceful race of telepaths who passively pick up on the minds of powerful creatures. As said creatures tend to be malicious evil, the flumph hates them and eagerly shares what information it has acquired through this brain-scanning to heroic adventurers so they can more easily defeat the monster.