What did I just see?

I need help on what I just saw at the 3 second mark in this video. If you look just to right of center, You'll see what appears in the tree line to be a bag or blanket or even spirit getting out from under a Grad attack. What makes this so unusual is that at the 5 second mark, it appears to be running on legs in the air.
So, take a look. Is it a Blanket? or a Bag? or a Bird? or a Plane? or is it Casper the Ghost Haulin Ass?

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