I am turning away from the Sea Rex theory because it was made a year ago from today. So in light of that I am turning to the notion that the creature could very well be a Meetle or a Mug. Because one Meetles and Mugs are that size and two they are land based creatures. The dried up river bed could be ancient and could not even be a dried up river bed.
We don't know. But the creature has legs much like a Meetle's legs. Almost insect like.
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