OK, item by item. Assumptions: "real" means the same as being INSIDE this particular universe. It circumvents the quasi-reality of "if you believe in them, they're real somewhere," which, while I believe it to be true, is not particularly exciting, because it makes them real without really being real. Kind of like my being a therian calling myself a werewolf but not actually shape-shifting by the light of the full moon so much as simply meditating on it.
Bigfoot: The sasquatch is a plausible possibility. The famous photo, unfortunately, is probably a fake; someone finally found by digital analysis a zipper. But, there's a convincing stretch of vacation photos from the Himilayans of a Yeti, and there is precedent for previously "mythical" species being officially "discovered." The gorilla was once thought to be myth and is pretty similar to some sasquatch descriptions.
Aliens: Mathematically almost inevitable, but elusive proof. We could very well end up finding microbes on Mars in our lifetime. As for beings like the Grays, I'm very mixed about them. On the one hand, there were a lot of sightings and accounts prior to their pop culture status. People who describe being abducted have reproducible signs of post-traumatic stress disorder, genuinely arguing that something happened to them. The lack of permanant physical evidence can be almost expected if you're dealing with a genuinely superior intellect that wants to hide proof of itself from us. (If anything, one can wonder why the entities like the Grays aren't doing a better job of hiding themselves, unless they want to be ambiguous--maybe a psychology study on human faith.) But, before you pull out the Betty and Barney Hill starmap and follow the A-5 to Zeta 2 Reticularis, there's also strong arguements against. The biggest is that human evolution took a lot of natural events to guide our specific shape, and there's no sign whatsoever that anyone else fudged us into our present form, and yet UFO accounts routinely describe "humanoid" creatures. Many abduction experiences even go as far as describing interbreeding with them, something that should be less possible than mating humans with garden spiders or coleus plants, both of which are at least distant relatives on the same planet. Legends of faeries and angels abducting people are very similar to UFO abduction stories today, and there are ways of using this observation to argue both for and against the reality of beings like the Grays, Nords, or "reptilians." I say it could go either way. My working hypothesis is that these beings are either Ultraterrestrials--visitors from another reality rather than another planet, or they're time travellers, genetically engineered from our decendants. But, they could be nothing at all but a rationalization of brain farts that cause people spontaneously to suffer PTSD. According to skeptics, that's the rational explanation.
Right...
Loch Ness Monster The best photos, showing us a modern pleisiosaurus, have recently been admitted to be a fake. But, there's a lot of other sightings, both of the creature in Loch Ness itself and of others like Ogopogo and Lake Champlaign's "Champ." How a dinosaur could go unnoticed all this time is a mystery, and it's unlikely that a single one could have lived for 65,000,000 years on its own, so there would have to have been a population of the beasties all this time. But, we've found things like the coelocanth, so it's plausible. Vacation footage of the Yeti...
Werewolves: Their plausibility / implausibility has been talked to death already. I'd generally have to say no. However, I'll confess I have no perfect explanation for the Beast of Bray Road in Wisconsin.
Witches: I know for a fact they're real, because I happen to be one myself, and so is my wife and many of our friends. Wiccans are followers of a nature-based religion that practices magic. By "magic" I'm referring to a combination of ceremonial ritual and creative visualization. It's an area where I'll openly admit I diverge from the flat skeptic's model of reality, because it would take an inordinate number of lucky coincidences to explain the effectiveness of magical rituals. (Some Wiccans and other metaphysicists use the spelling "magick" to distinguish our ritual-induced manifestations from other uses of the word "magic," like basket ball stars and a highly addictive trading card game.) As far as the "devil" witches of the middle ages, that was created out of paranoia and ergot-contaminated rye. ("Well, we did do the nose. But she is a witch.")
Faeries: I do believe in faeries! Explaining what they are, however, is purely conjecture. But, they're appearantly something from another realm that have the ability to affect this realm. Could possibly tie into some of my working theories about alien sightings.
Dragons: I just saw a great special on Animal Planet, discussing a hypothetical real-world evolutionary dragon. (Apokryltaros / Avacnea, it would have been right up your alley.) But, most likely, if they're real, they're probably also somehow affiliated with faeries and aliens. Like werewolves and other shapeshifters, they do seem to be a universal symbol in human consciousness, appearing in every human culture, even those in places where there are no reptiles. So, they couldn't have been just an explanation for dinosaur bones.
Unicorns: Probably started out as a myth that grew. Although matching their cool powers will take a long time, we should be able to engineer through genetics a horse with a horn in a few decades, meaning, ethicists willing, I could some day ride a horse that looked like one.
Griffins: Probably born out of explanations for large bones, though I'm open to less mundane possibilities if given evidence.
Vampires: Similar to therianthropy, there's real-world counterparts with metaphysical ties, but I am highly skeptical of the idea of there actually having been an immortal race of beings who feed on human blood. Accounts of bodies rising can be explained by decomposition anomolies, as well as the reluctance of grave robbers to admit getting into tombs.
Ghosts: I've seen one, or at least the effects of one's presence. I've also seen another show up on developed film, though at the time it was not perceivable. So, I know they're real. I've made no effort to show off the photo to
Ripley's Believe It or Not or the like, because I don't feel a need to try to convince others. I'm therefore not asking or expecting anyone to take this testament as credible. I'm simply saying that
I believe. Working hypotheses involve either electromagnetics or the concept of souls and the astral plane as an extra dimension.
Zombies: Folklore surrounding them stem from Haiti, where unscrupulous persons claiming to be witch doctors have given people catatonic drugs and pretended to resurrect them from the dead, for a fee. To this end, zombies that are literally undead are almost certainly non-existant, but like vampires, werewolves, and witches, there is a real counterpart.
Elves: Harvey's Pooka Dictionary, 3rd Edition: "see faeries." Faeries: "see elves."
...No.
Let's keep the insults to a minimum, (Minimum=0), mmmkay?...
(now if I could only play god and mix human and wolf dna to make some crazy sci-fi fantasy into life *snicker* 
OK, then i misread something. Yeah, I read an article in scientific American about the thingie in titan (yesterday, in fact). I probably automatically linked it to europa. But yes, ppearently europa is the one with an underground body of water which may have sustained/sutain life.