Could be the never written sequel to S.M. Sterling's "Conquistador".
Oakland, 1946. Ex-soldier John Rolfe, newly back from the Pacific, has made a fabulous discovery: a portal to an alternate America where Europeans have never set foot—and the only other humans in sight are a band of very curios Indians. Able to return at will to the modern world, Rolfe summons the only people with whom he is willing to share his discovery: his war buddies. And he tells them to bring their families....
In 2009 the portal was destroyed. Trying to reopen it they finally succeed, but when the newly created portal opens:
A long boom swung through the gate, with sensors on its end. And a television pickup; it was keyed to a large flatscreen placed where they could all look at it.
The screen flickered, then settled to a clear image. It was raining there, too; as well it might, in midwinter along the Californian coast.
"But where is the Gate complex on FirtSide?" he asked himself; all he could see was long grass....
Rolfe began to laugh; coughed, recovered, laughed again.
Because in the grass was a dead animal, huge and shaggy, almoust certainly a giant sloth. Paws braced on it, the sabertooth bared its foot-long fangs and screamed, flattening its ears and bristling its orange-and-black-striped fur.
[End of Conquistador]
I waited for a sequel, but nothing.
Could be he came across the story you are seeking and decided to not write a sequel to Conquistador.