It certainly could have been touching the ground when it pulled out the pilot, just like it appeared to be when killing Eko (see link above) – it just looked like an arm coming up from the ground, even though it reached very high.

Same for the top of the trees, etc. – it can reach very high, but not totally disconnect from the ground, which it would need to do to clear that fence.

Also in the first episode the top of the trees were ruffled.

My theory is there are multiples and they collect into one. It doesn’t make sense that they could fly but they need to explain the intances were it wasn’t.

I do think it’s tied to the ground and is mechanically controlled from something in the ground. That’s why it pops out of the ground and tried to pull Locke down.

OK, I just went and watched it, and there’s no part of that scene that suggests the “monster” isn’t anchored to the ground the whole time.

I’m not saying it definitively CAN’T fly, but that certainly would explain why it couldn’t go over the barrier. Anyone’s welcome to point out a segment where we do see the monster “flying”. I’m not counting frame-by-frame only references or fleeting passing by of what appear to be smaller versions either (since I think most of those are still pretty close to the ground, even if the whole cloud isn’t touching the ground).