It's some sort of enormous creature seen only by one of the castaways, thusfar, the oddball outdoorsman Locke and he didn't tell the others about it due to his "bonding with nature philosophy". Series producer J.J. Abrams intends to keep it a mystery for some time to come; all he's said so far is that it's NOT a dinosaur. All we know is that it's really heavy, as its pounding footfalls suggest, that it's at least 30 feet tall to make the trees move as it does and that it's a killer, having ripped the doomed airliner's pilot right out of the cockpit windscreen and skinning him alive. For some reason, it didn't kill the somewhat wacky Locke in his encounter with it; it wasn't explained why. Try not to rationalize this too much, remember, this is also an island where on two occasions so far, a kid read a comic book featuring a polar bear then a REAL polar bear showed up shortly thereafter. Yes, a polar bear on a hot tropical island! I hope I didn't spoil the story for you by revealing that, if you haven't yet seen it. I love the show because of such bizarre elements; just don't look too hard for a logical explanation of what's going on; fan speculation on such is looking at all sorts of possibilities. When Abrams decided to produce the series, he decided to add some weird stuff to what was originally to be a very straight-laced "Survivor" type drama to give it added intrigue. This has worked, in addition to having a large cast of compelling characters, to make "Lost" one of TV
's hottest new shows. I'm not sure they've yet decided whether to reveal the monster by season's end but at some point, I think they'll have to and create a new mystery to keep the show fresh.