On the Gelatinous Cube
Let's admit it: it's probably the best named monster in the Monster Manuals, the various compendiaries of mystical beasts used in Dungeons & Dragons. Maybe the wyvern for its cryptic spelling. Or the rust monster, also a favorite in my teenage games. If you attack the rust monster with most metal weapons, well, you can imagine. If you attack the gelatinous cube with most weapons, it can subsume them. There is a chance.
Apparently a creation of D&D creator Gary Gygax, it is a sweet monster, a 10-foot x 10-foot x 10-foot cube, partly because there is no getting away from it. In fact, fighting it is a conundrum for players who are used to just bashing anything they come across. It is a conundrum. How intelligent is it? See Ed Greenwood's article, "The Ecology of the Gelatinous Cube," in Dragon Magazine #124 (TSR, 1987). Or come by my place if you want the dissertation. Or read my essay "Geas" in Vanishing Point (or in the literary journal Fourth Genre, if you want a preview, though that version differs from the book version) which tracks some of it.