Stephen King's The Mist Wiki has a more comprehensive article on the same creature. For it, see http://stephenkingsthemist.wikia.com/wiki/Behemoth
A very large creature from another dimension (possibly Todash space), with six legs. Other than the legs, with hundreds of the aforementioned small flying creatures attached to them, this creature is unseen. Although the creature's exact size is never specified, David gets the impression that its size would make a blue whale resemble a trout if both were posed together, and its weight is sufficient to leave six-foot-deep footprints the size of a large SUV in solid concrete.