This thread is awesome! I love variant ideas for monster origins, it really keeps players on their toes and maintains a sense of wonder.
For my current game, I lifted an idea from another forum about the connection between goblins and hobgoblins. In Alaria, hobgoblins are one of the True Races of the world: organised and intelligent to the extent that they have their own settlements, industry, systems of writing and numbers etc. Their lives are harsh, rather violent and highly regimented in some ways (think Norsemen) but they're fully-fledged "people" with a lifespan of around 40ish years and sometimes longer. However, they suffer from a peculiar biological issue whereby they only reach full maturity if they undergo a major adrenal shock of some kind. This causes a problem as the hobs get steadily more civilised.
In order to maintain their population levels, low-status hobgoblin females are forced into a role as "breeders" and produce lots and lots and lots of offspring. These offspring are looked after in group nursery environments until they reach around 5-6 years of age, at which point the entire age group is driven out of the hobgoblin settlements and outside their lands under threat of violence. These hobgoblin children, forced to live without support, quickly develop into bands for survival. The regions between hobgoblin and human settlements are filled with lots of these desperate, starving children, and humans call them "goblins".
Many goblins die quickly. The ones that survive usually only remain "goblins" for perhaps 3 or 4 years before a near-death experience in combat, fire, or any other of the myriad dangers of the wilderness force them into the adrenal shock they require to mature. As they grow prodigiously and develop the characteristic long brow and intelligence of a hobgoblin (goblinoids do not have animalistic features in my setting, since they are not beastmen) the changing goblins quickly realise their situation and journey back to hobgoblin lands. Hobgoblins find it easy to recognise adults by their scent, so the large majority are allowed back into the hobgoblin settlements.
A very few goblins do not undergo the physical changes even after surviving a near-death situation and the ensuing adrenal shock. These unfortunates continue to grow, though they remain immature, and their increased stature and hard-won experience marks them out as leaders amongst the squalling masses of goblins. These form the goblin Champions and Chieftains; few if any ever master magic of any form, so goblins (unlike hobgoblins) have virtually no Witch Doctors or Shamans.
Orcs are a peculiar aberration. The connection between goblins and hobgoblins is poorly understood, but some sages have recorded it. As a result, at some point in history evil spellcasters have taken advantage of the unique hobgoblin process of maturation. A spellcaster oi high enough level to create crossbreeds may construct a Gall, a fleshy cage-thing into which captive goblins can be inserted. After a period of screaming and crunching sounds, the goblin is forced through a magically-controlled adolescence and transformed into an Orc - larger and far more muscular than a hobgoblin, with their long lower canines transformed into tusks and their intelligence mostly maintained, but their ambition dulled powerfully in favour of a rather easily-led nature. These Galls are primarily used by evil spellcasters to create armies of loyal creatures that know nothing except service to their dark masters. (And yes, this bit was definitely inspired by the movie version of Uruk-hai.)
Orcs have Champions and Chieftains, but no Shamans or Witch Doctors. A very rare mutation or error in the Galling process produces an unusual orc with greyish skin and smaller stature; these are Grey Orcs and share the same natural spellcasting capabilities as Elves have in my setting. While they are rare to begin with and most die early in their lives, my setting has a rare one or two Grey Orcs survive to high enough levels that it is possible for them to create Galls of their own, producing more Orcs.
So far, my players have been loving discovering all this. I suspected they would be a little squicked to discover that those goblins they slaughtered with impunity at low levels were actually desperate children, but not really - if anything figuring it out has only made them more determined to smash every hobgoblin they see to pieces; which is ironic, given it's probably easier to deal non-violently with hobgoblins than any other "beastmen" (only a few humans understand the difference).
(For reference, Ogres, Trolls, Gnolls, Bugbears and Kobolds are all still "animal crossbreeds created by evil sorcerers for their dark purposes" as per usual ACKS. The evil sorcerers were Nazi Elves... maybe that's a story for another time.)
This is novel-worthy. Seriously - awesome.