The Ruum

"The Ruum" is a 1953 science fiction short story by American writer Arthur Porges.
Plot summary.
A shape-shifting, indestructible robot—a "Type H-9 Ruum"—is accidentally left behind on earth during the age of the dinosaurs by visiting aliens, and not recovered because their ship is destroyed in a battle. The Ruum's work is collecting different species of animals within a 30-mile radius, weighing 160 pounds (give or take 15 pounds) and preserving them in a state of suspended animation for later retrieval.
Many millions of years later Jim Irwin, a uranium prospector in the Canadian Rocky Mountains, encounters the Ruum's specimens, animals which include modern and prehistoric mammals and a small Stegosaurus. After he has noticed that the animals are all of a uniform size, and alive, the still-functioning Ruum appears. Shooting it with a rifle does not damage it, and when the approaching blob-shaped Ruum reveals gripping hooks and a probe dripping green liquid, Irwin runs as it pursues him at a steady five miles per hour. It is noted that Irwin weighs 149 pounds.
Fit and military-trained, Irwin attempts to evade and destroy the Ruum. First he hides above his trail and drops a boulder on it, but this also fails to damage it. Then he climbs a sheer cliff, but the Ruum extrudes a climbing claw to the ledge where he is hidden. He tries to dislodge it with a stick, and finds the Ruum can generate an electric shock. He shoots it and dislodges it, but the Ruum is unharmed by the fall and responds by growing multiple claws. Having not had any time for food or sleep, Irwin eventually circles back to his camp by a lake and sets a trap with dynamite. Taking advantage of the Ruum's distraction by a grizzly bear, which it kills, he detonates the dynamite using a pistol bullet.
Dazed by the explosion, he sees the Ruum still unharmed and still approaching. Exhausted and defenseless, he awaits it adding him to its collection. However, it lifts him up, then puts him back down and leaves. Soon afterward, his colleague Walt Leonard arrives by seaplane and rescues him. Safely in the plane, they make plans to bring a helicopter and collect some of the Ruum's specimens in its absence to sell to scientists. Leonard comments on Irwin's haggard appearance, guessing him to have lost 10 pounds during the chase: the reader understands that this caused his weight to become less than the Ruum's weight criterion.
Sequel.
A sequel to "The Ruum", entitled "A Specimen for the Queen", appeared in the May 1960 edition of "The Magazine of Fantasy and Science Fiction".
Reprints.
"The Ruum" was selected by David Drake for his 1988 anthology "Things Hunting Men," where he commented that this was one of Porges seventy-some "meticulously crafted stories" where "sometimes the problem the protagonist faces really is insoluble." But, here, Jim Irwin never gives up.
Many other reprints are listed at the ISFDB.