odd question, things like Sasquatch and the lock ness monster it's iffy. i'v never seen them, but i have a few friends who have seen a Sasquatch. i think there's still many undiscovered species, so i don't rule anything out, but i do know the difference between fantasy and reality, so i'm 95% sure they're myth [more or less depending on the creature]
other things like dragons and mermaids are astral beings, and those could be argued as imagination if someone doesn't believe in the astral. physically though, no.
how to find such creatures, same way you'd catch a rabbit or bear, know them. if you find a certain creature specifically in the Yukon, then look for it in the Yukon. if they eat salmon, pick up salmon to bate traps, or an areal where you'll be waiting for them. so, as usual, research like crazy, then if you want to track down a Pegasus or whatever, do so, just don't get your hopes up, and bring a good camera and a tripod so we don't get another shaky blurry photo of the moth man.