Anything is possible, and dare I say probable. Its a great big wide world. There are so many unexplored caves, caverns, places that have probably been buried by earthquake, erosion, etc, etc. There are, no doubt many creatures we probably don't know about, big and small. The oceans take up most of the world. Surely much of it has been mostly unexplored. There are just places we as humans haven't been able to reach yet. Regardless of how much like the serpents of lore the beasts may look, it seems unfair to think they might not exist because nobody has gotten a really good look at one. There have been sightings and some (although not very clear) pictures...just because the exact description is vague, doesn't mean they don't exist.