Let's join the life cycle of the bugbear with our little friend, the goblin. Goblin children, about the age where human children might be called toddlers, are notorious for getting into places and situations away from their parents. They are also notorious for being horrifically violent, cruel (as in physically cruel) and deceptive, but that's not important right now.
In some areas near where goblins dwell, the little goblins sometimes find themselves (either by misadventure or design) in proximity to a certain breed of enormous semi-translucent spiders.
|Sort of, but 8 feet long and way more gross.|
The little goblins are bitten, envenomed, and swallowed whole by these spiders, which have big sacs, one sac per spider, inside their abdomen. The little goblins languish comatose in the spider’s translucent abdomen as they slowly transmogrify into full-size bugbears. As they grow, they gradually take over the spider’s body, slowly killing it as their limbs squeeze out into the four rear legs of the spider, expanding the girth of the legs and forcing them to moult aggressively.
In the final stage of the process the spider takes on a centaur-like existence, with the bugbear occupied abdomen (with their head in part of the thorax) acting as the horizontal motive portion, and the rest of the spider vertically upright, it’s non-bugbear-filled front legs flailing in pain as it continues to search for sustenance to feed the growing bugbear within. Finally, the fully grown bugbear bursts from the still-living spider, killing it. The bugbear will, over time, produce spider eggs about the size of green grapes. These eggs generate in glands analogous to a dog or skunk’s scent glands. You can probably guess where these nasty, slimy, translucent eggs can then be found, and what the hatchlings' diet consists of.