'Never say never, never say always,' is my policy when considering unexplained natural phenomena. We still know very little, for example, about living things at the bottom of the deepest parts of the ocean. When we discovered the sulfur-based tubeworms living around seismic vents, scientists admitted they'd never imagined life could take a form like that. Who's to say we'd be able to recognize life elsewhere in the universe if we saw it?
Ghosts? Ghosts have largely been understood as non-corporeal beings of a spiritual nature. As such, they'd probably be better explained (if in fact they do exist) in the language of metaphysics than in terms of empirical science. At least that's how I see it.
And every society has its monsters. They're elements of every culture's mythology, and they often represent the less-desirable elements of humanity. Rarely are they taken as seriously as the Loch Ness monster or the Abominable Snowman (mythology exists primarily for the purpose of instruction after all), but most myths start out at some level with some minuscule amount of fact. In that case, I'm sure there might be something living somewhere in Scotland that doesn't fit existing taxonomy, but what exactly is it? I have no idea. And maybe it's better that way.
