Gelatinous Icosahedron

 When I think back to my earliest days playing D&D, the monster I think of most isn't the dragon. It isn't the orc, the gelatinous cube, or the ogre. It's the goblin, from the Basic Set, naturally. Goblins are stated to be small, incredibly ugly human-like creatures. How small, exactly, isn't specified; they could be dwarf-size, halfling-size, or smaller, depending on how you like your goblins. The entry does tell us that they have red eyes that glow in low light, and pale earth-colored skin, "chalky tan" and "livid gray" being given as examples. They're identified as avowed underground-dwellers, with infravision to a 90' range. Full daylight hurts their eyes, and they fight at a -1 penalty. They hate dwarves so fiercely that they will always attack them on sight, so no negotiating with goblins if there's a dwarf in the party! There's also a 20% chance that one in four goblins encountered will be riding dire wolves, which substant...

 It's been said that gnomes never really found their niche as a player-character race in AD&D, but as an NPC/monster type in Classic D&D I've always had a soft spot for these guys. To be sure, there is some overlap with dwarves, and to distinguish the two some sources have taken gnomes in a more comedic direction than I would like, but done well they've fit quite nicely into my own campaign settings.  Regarding gnomes, the Basic Set says that they are related to dwarves but are smaller. Whether this means shorter, more slender, or both is not stated, but I generally picture them as being both shorter and less stocky of build than the typical dwarf. Like dwarves, they sport full beards (at least the males do; nothing is said of gnome-women, so indulge your own preference there) but are also stated to have long noses. While dwarves are typically dwellers in rugged mountains and deep caverns, gnomes are explicitly stated to prefer burrows in the lowlands. In theory, at...

 Some D&D monsters are adapted from myth, legend, and folklore, some are taken from literature and fiction, and some seem to be wholly original to the game. The gnoll belongs mostly to this last category, having no direct mythological analog that I'm aware, and seemingly bearing only a passing phonetic resemblance to Lord Dunsany's gnoles. Gnolls are rumored to be the hybrid creations of an evil wizard (a well-worn fantasy cliche, but also a timeless classic) combining gnomes and trolls. The truth of this is left to the DM and his or her particular campaign setting, but where it is true, the gnolls really got the shaft, having neither the gnome's industrious intelligence nor the troll's regenerative powers. They're basically big dumb brutes, higher up the humanoid food chain from orcs and goblins but beneath bugbears and ogres. Even so, it's a niche that needed filling, and the gnoll is a pretty good fit.  With an AC equal to mail armor, a stout 2 Hit Dice, ...