any theroies about the hole in the ground

The "hole" story seems to be based on Devil's Hole Cave, where a reptilian monster called the Gowrow supposedly dwelt:

http://www.prairieghosts.com/boonec.html

Vance Randolph's 1951 book We Always Lie to Strangers is the main source of the Gowrow story. He also wrote of other reptilian critters in the Ozarks. The Jimplicute “was a kind of ghostly dinosaur, an incredible dragon or lizard supposed to walk the roads at night, grab travelers by the throat and suck their blood.”

The High-Behind was “a lizard as big as a bull, whose hind legs were ten times longer than its forelegs. This creature, according to some children I met near Big Flat, Arkansas, lies in wait for human beings on the trails at night and ‘laps ‘em up like a toad-frog ketchin’ flies.’”

Yet another reptile monster was the kingdoodle or whangdoodle (where do they come up with these names?). Randolph himself and a ‘possum-hunting friend came close to the whangdoodle near Waco, Missouri, when, as he writes, “I heard a strange booming sound in the woods along Spring River. ‘What’s that noise?’ I asked. The possum-hunter listened for a while. ‘I don’t know,’ said he, ‘but it sounds like a whangdoodle a-mournin’ for its dead.’”

. . . Yet I'm glad we weren't shown someone in a rubbery Godzilla suit or something in Encounters with the Unknown. Whatever-it-was in the hole was totally unknown, and all the creepier for it! The monster-noises were pretty effective; I totally forgot until a recent viewing that "it" eventually became so loud that the people in the nearby town (and the boy's farm) can hear it.