The sound effects were amazing, and it's clear you knew the feeling you wanted to convey with each part.
However, like most people said, the monster looked a bit silly. He was still kinda intimidating in my eye, but I really think its appearance could have been improved.
Aside from that, maybe it's just me seeing too much into this, but the woman seemed to me like a really bad mother: letting her daughter wander off, not being alarmed by the buffalo running away all of a sudden, just looking down at her son and crying (or something) as the monster is coming to get him, letting herself be fooled by a *completely white* duplicata of her son, etc.
Anyways, the style was good, although it could be improved upon, the idea was great, and the sound effects were amazing. The animation needed a little more work, and the monster could have been more threatening.
All in all, a good flash.