Bigfoot and Yeti .... nope.
UFOs ...... There are 70 thousand million million million stars in the universe (7 sextillion) and without doubt billions of planets, many of which could sustain life but perhaps not as we know it. Do aliens exist? IMO, yep, they do.
Have we genuinely seen them? Don't know. But consider the distances ....... our nearest star (Proxima Centauri) is 4.7 light years away from us and that's travelling at 186,000 miles per second every minute and every hour and every day. Exoplanets are (around) 50 light years away.
Assuming an alien craft can match the speed of light, I don't see how it's possible they would take the journey unless they have life spans far longer than ours. It would take us (with our technology) thousands (many thousands) of years to get to an exoplanet.
.
Maybe the speed of light isn't the fastest form of travel ....... both Hawking & Einstein have proposed how both space & time may be 'warped' and massive distances covered almost instantaneously. On the face of it this sounds unbelievable but no more so than telling a neanderthal man sitting on a log floating on a stream that man would one day be capable of travelling at six times the speed of sound (sr71 spy plane)
For Bigfoot & Yeti, et al, these are usually seen in tight geographic areas ....... now bear with me on this
Now, lets say that where we live, here & now, our 'reality' is the 'ground floor' of a tower block. Other realities are stacked above us. The reality where the Bigfoot live, one where the yeti live, much like a tower block with thousands of inhabitants.
You can't see all the inhabitants at the same time
You can (and do) see one or two, briefly, when the lift doors open. Some may board the lift with you and travel a few floors, then they leave again, or you get out at your desired floor, leaving them.
I believe that the places where 'mythical' beings are seen are places where our realities converge, if only for a brief moment, such as where our individual human lives, usually strangers', converge for a brief few moments whilst they travel in the lift - they don't always exist just as one can board & travel an empty lift without seeing a soul
Perhaps the reason we haven't learned much about these other creatures is because everyone travelling in a lift rarely talks to anyone else

