You read my mind. I was thinking of the Amur tiger. Korean researcher Sooyong Park spent months in the Siberian and Chinese forests, eating rice, nuts and salt, and removing all traces of his own poo, hiding in a hole underground for years to film them. Yet we have extensive footage of these animals and their family life. And they had been seen and captured before.
Also, bigfoot must be the only primate species in like 55 million years that's not highly social, curious, extremely boisterous. Isn't that peculiar?
And lastly, we're aware of the existence of a ancient species of ape (the Denisovans) thanks to a tiny fragment of a phalanx from the pinky of one hand, including sequences of their DNA. And we can't obtain definite proof of the existence of a big hulking ape who inhabits the Earth now?