Slime
The Slime, also known as the Ooze, the Gel, the Jelly, or the Goo, is a humble form of monster that pops up in absolutely every frigging fantasy setting you can imagine, and even a few science-fiction settings. It is most famous in tabletop games for its many diffuse forms in Dungeons & Dragons and in videogames for Dragon Quest (which likewise has a fucking huge array of possible slime-forms), but, like we said, you can find a slime just about everywhere if you look.
Slimes are usually low to middle tier threat levels; they are mindless masses of animate sludge, and some higher-level variants may be made of elemental matter, such as water, "liquid ice", magma, molten steel, etc. They have no culture or higher purposes, they just ooze around eating anything organic they touch and growing bigger until they have to divide. Hardly likely to outwit most adventurers, but many games make them fairly resistant to certain kinds of attack, especially physical ones, so just assuming they're harmless is a good way to get dissolved. One prominent commenter described fighting a slime as "playing a terrifying game of 'guess the immunity'," referencing to the fact that unless you have your appropriate Monster Manuals memorised, you can rarely predict what will kill a slime variety outright. And guessing wrong can sometimes be worse than not trying, because using the wrong damage type can cause the slime in question to divide.
Surprisingly, slimes have long been given the monstergirls treatment; Slime Girls (also known as Goo Girls) are typically not very smart, but very affectionate and horny, and their gelatinous bodies have a lot of kinky shit they can do in the bedroom.
Perhaps the most notorious slime to come out of the tabletop game field is the gelatinous cube, a D&D monstrosity that takes the form of a huge cube-shaped mass of near-translucent gray or green jelly, perfectly sized for oozing through the typical dungeon corridor as a living, insurmountable barrier. Like the Rust Monster and Owlbear, it's one of those absurdities that everyone pokes fun at, but which has too much nostalgic fondness from the fanbase for anyone to seriously consider getting rid of it. Later, fortunately, it did come with a half-assed explanation: wizards breed them as living janitorial services to clean the garbage out of the perfectly-square corridors of their evil dungeons. As it's also mostly-transparent, save for the dissolving bits of armor floating in it like fruit in a jello dessert, walking straight into it only to be engulfed is an occupational hazard for dungeon-delvers.