The idea of most "ultra terrestrials" being offshoots of beings from Earth from multiple futures has long seemed reasonable to me.
Their numbers and vitality would be fluid in regards to changes made in their pasts (our present). The Cs have also hinted that this idea is very close to the truth.
Say we were facing an existential issue in the future. Let's say we reached a genetic / mental dead end.
Some groups would undoubtedly go back into the past to meddle, thus creating alternate futures for their race to inhabit.
After creating some new hybrids or planting new genetic information in the population, you check to see which new futures you now have access to.
Similar to exploration and discovery of new land to settle, except the new land is new realities.
It's also easier to come to terms with the existence of intelligent reptilian hominids when you begin to consider that perhaps there is a reality in which there was no asteroid impact.
Dinosaurs continued to walk the earth for millions of years until an intelligent lizard species emerged. When they were advanced enough to go back in time and experiment with Earth's biome, they created a reality / found one, in which dinosaurs are extinct and mammals are more prevalent. Then begin to design their new food sources.
This is interesting because it hints that what we do in the present affects which of these races would frequent us more. And "who we are" as individuals and groups would likely affect which groups we would likely be entangled with.
