Bigfoot: interdimensional being or super crafty woodbeast?
Neither. Bigfoot is exactly as intelligent as us. We walked in the daytime, they walked in the nighttime, our nocturnal siblings in the shadows. But while we were still wandering hunter-gatherers, they
changed. Like we would, a few thousand years later. Tools. Weapons. Agriculture. Domesticated animals. Stable settlements. As humanity blinked in the Pleistocene sun, Bigfeetses's population exploded across the night. They blanketed the planet in the tens of billions.
They made things that we still can't comprehend, even though we've thoroughly studied the surviving pieces. Organic technology. They made trees and birds of prey grow into fast-moving ships, herds of animals that became trains, bushes that became flying vehicles. From insects and pigeons they made things equivalent to cell phones, televisions, computers. Atomic bombs. The Children describe vast shining cities, stretching across glaciers and penetrating the deepest caverns, grown skyships of ivory and spider-silk, creatures tending them with hundreds of blinking eyes.
We were rare, like gorillas now, a few hundred thousand left at best. We avoided their settlements just like wild animals today avoid ours. Bigfeetses understood we were intelligent like them, but avoided us just as we avoided them, saw us as fairies, as gnomes, ascribed us supernatural powers, said we ate bad children while they slept in daylight. They fenced off our dwindling wild populations in conservatories, outlawed poaching but in the underground consumed our bones as aphrodisiacs.
Then their civilization fell. And we did it. By 'we' I don't mean the Foundation. By 'we', I mean humanity.
The story is muddy. Supposedly a trickster forest god showed humanity favor, showed us the master's tools and how to use them. Why we did it, we don't know. Perhaps they hunted us, perhaps we were simply afraid. Perhaps it was just that they fenced us in, unintentionally or not. We simply don't know what the truth is. Somehow we acquired Bigfeetses's own technology, and with it, we instigated an SK-class dominance shift in which humanity became the dominant species of Earth.
We wiped out 70% of Bigfeetses's population in a single day. The Day of Flowers, the Children called it. Supposedly every flower bloomed that day, while our enemies died in their sleep. Then we hunted the rest down. But we went further than just killing them. With a few of the more twisted of Bigfeetses's devices, we drove the survivors mad, even those hiding beyond our reach. We trapped them in their own minds, blocking higher functions and leaving their bodies to fend for themselves like any ordinary ape. We slaughtered their living machines and burned their vast shining cities with Bigfeetses's bioweapons that reduced everything to slurry and dust that washed or blew away in spring rain and wind.
We left no traces. Not even our own memory. We turned one of the weapons on ourselves, wiped out any knowledge of SCP-1000 and the greatest civilization the planet had ever seen. Only a few humans protected themselves from the effect, kept the forbidden knowledge, just in case. The rest of us went back to being hunter-gatherers, none the wiser.