The basic plot of this (and I'll try not to spoil anything as best I can), there's this man who discovers a really well-preserved baby stegosaurus and, upon further inspection, realizes it's alive. He's shocked by this and then realizes there are a bunch of other animals, from different time periods and all about the same size, that have been frozen and kept alive for all this time. Then an alien pops out of nowhere and chases him and tries to add him to the collection. It's a really good sci-fi suspense short story and I want to show it to my friend but I cannot for the life of me remember what it's called. Please help! Thank you so much!
"The Ruum", a short story by Arthur Porges, also the answer to the old question What short story has a ball chasing a researcher to add him to its collection?; first published in The Magazine of Fantasy and Science Fiction, October 1953, available at the Internet Archive. (A sequel, "A Specimen for the Queen", appeared in The Magazine of Fantasy and Science Fiction, May 1960, also available at the Internet Archive.)
It was like some enterprising giant's outdoor butcher shop: a great assortment of animal bodies, neatly lined up in a triple row that extended almost as far as the eye could see. And what animals! To be sure, those nearest him were ordinary deer, bear, cougars, and mountain sheep — one of each, apparently — but down the line were strange, uncouth, half-formed, hairy beasts; and beyond them a nightmare conglomeration of reptiles. One of the latter, at the extreme end of the remarkable display, he recognized at once. There had been a much larger specimen fabricated about an incomplete skeleton, of course, in the museum at home.
No doubt about it — it was a small stegosaur, no bigger than a pony!
Fascinated, Jim walked down the line, glancing back over the immense array. Peering more closely at one scaly, dirty-yellow lizard, he saw an eyelid tremble. Then he realized the truth. The animals were not dead, but paralyzed and miraculously preserved. Perspiration prickled his forehead. How long since stegosaurs had roamed this valley?
The alien collector is a spherical robot:
Jim Irwin had once worked with mercury, and for a second it seemed to him that a half-filled leather sack of the liquid metal had rolled into the clearing. For the quasi-spherical object moved with just such a weighty, fluid motion. But it was not leather; and what appeared at first a disgusting wartiness, turned out on closer scrutiny to be more like the functional projections of some outlandish mechanism. Whatever the thing was, he had little time to study it, for after the spheroid had whipped out and retracted a number of metal rods with bulbous, lens-like structures at their tips, it rolled towards him at a speed of about five miles an hour. And from its purposeful advance, the man had no doubt that it meant to add him to the pathetic heap of living-dead specimens.