|This article or section is about something oldschool - and awesome.|
Make sure your rose-tinted glasses are on nice and tight, and prepare for a lovely walk down nostalgia lane.
Demihuman is a term adopted into the /tg/ lexicon from Dungeons & Dragons prior to 3rd edition. It referred to "those nonhuman humanoid races you can play in-game" - that is, those in such parties as Tolkien would assemble, the elf, dwarf, and halfling. Other races, such as orcs, goblinoids, kobolds, xvarts, ogres and so forth were all lumped together as basic "humanoids" which you had to fight. AD&D also allowed the gnome (which was fine) and the half-orc (stretching the concept).
Somewhere around here are the super-SUPER Mary Sues, even over elves (if the setting has those). These are giant races who live among men (sort of). These rule us, or try to rule us, or used to rule us, or some combination of same. In Jewish lore, those would be the nephilim. Dragonlance, which started as a Latter Day Saint expy, perhaps-unsurprisingly pioneered this with the Irda. Since then we've seen Raymond Feist's dragon lords aka Valheru, Stephen Donaldson's giants (as expied in Arcana Unearthed), the Deverenians in Larisnar, and Avadnu's Sulwynarii. At Epic level: LeShay. Nobody human much wants to play a human in a nonhuman-dominated setting, so most such settings end up quarantining their nephilim somehow.
With the release of 3rd edition, this term was dropped from the official D&D lexicon. Some suspect worries over its hidden assumptions; using "demihuman" to refer to playable humanoids could be taken to imply that 1) all civilized/nonevil humanoid races are related to humans, and 2) those races are somehow inferior to "real" humans, while "humanoids" are shittier still. Note that neither of these assumptions are at all true in most D&D settings, despite what most editions' crunch would have you believe. And anyway even before 2e, Basic Dungeons & Dragons had GAZ10 allowing for full-on orc PCs, and hobgoblins and gnolls and whatever else. That ship had sailed.
Still, the term does occasionally pop up; aside from the obvious nostalgia appeal, the plain truth of the matter is that "demihuman" is a simple shorthand way of referencing all of the "traditionally non-evil humanoid races" with a single word. And what fa/tg/uy is opposed to the idea of having a shorthand name to call something, really?
Anime has resurrected "demihuman" for monstergirls, especially for human-mammal mixes like kitsune and catgirls. Minotaurs would probably count but since they're bigger than us, they don't usually feature.