Cryptomundo
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A Western mountain lion. Wildlife officials say they think sightings in the East have been Western cougars that wandered eastward, freed pets or a Florida panther. Photo: National Park Service/AP

Diana Marchibroda insists she saw the beast near the Appalachian Trail in Virginia in May. From the woods sauntered a “tall, very sleek” mountain lion, she says. Ms. Marchibroda, a dentist who is 62 years old, says she and her silver-haired miniature schnauzer, Sophie, “both watched in awe.”

“My sighting is ABSOLUTE,” she wrote the U.S. Fish and Wildlife Service in July. “I know what I saw.”

Dozens of similar missives have poured into the agency as it proposes removing the Eastern mountain lion from the list of endangered species, where it has been since 1973. That change comes because the agency believes the creature no longer exists and would effectively render the subspecies extinct.

The roar of protest is from Easterners who contend the formidable felines still roam forests, fields and backyards from Maine to Georgia.

“There was no mistaking that long tail!” wrote one commenter to the agency in June, about an alleged sighting in New York. “Big as my bike,” promised another about a purported lion in Harrisburg, Pa.

The debate is “sort of in the realm of Bigfoot,” but with more scientific basis, says Noah Charney, an expert animal tracker in Western Massachusetts. The occasional mountain lion is spotted in the East, after wandering in from the West, but it is exceedingly uncommon and officials say people are reporting far more sightings than technically possible.

Also called cougars, pumas and panthers, mountain lions boast impressive tails, buff builds and often tawny-brown coats. They thrive in the West and have expanded eastward in recent decades, breeding in spots like South Dakota, with sightings increasing in the Midwest.

Though scientists debate whether there are genetic differences among subspecies of mountain lions, federal officials say an Eastern branch of the cougar largely disappeared in the 1800s, killed by early settlers defending themselves and their livestock. The agency says the last known Eastern mountain lion was trapped in Maine in 1938.

“We’ve looked and looked,” says Mark McCollough, an endangered species biologist who led the cougar study for the Fish and Wildlife Service, which in June proposed to “delist” the Eastern mountain lion from the roster of endangered species. “This is not something we do gladly or feel good about.”

This is touchy terrain. The demise of the Eastern cougar is a long-running topic that has stirred a “whole cougar phenomenon” of bloggers, wildlife enthusiasts and even tricksters endeavoring to prove otherwise, says Mr. McCollough.

“I can’t think of any other animal that has captured the imagination of the public in the way that the Eastern cougar has,” he says.

Cougars do show up in the East now and then. But after studying reports going back decades, wildlife officials concluded these sightings were either Western cougars that wandered east, freed exotic pets or a Florida panther.

Confirmed cougar sightings east of the Mississippi River are “quite rare,” happening perhaps every five years, Mr. McCollough says.

Yet, wildlife agencies in the East receive hundreds of reported sightings of mountain lions each year, particularly claims of black panthers, which have never been authenticated in the U.S., the Fish and Wildlife Service said in its review.

The vast majority of sightings turn out to be bobcats, bear, deer, fishers (an animal related to the weasel) or domestic pets, the agency concluded.

David Kocka, a Virginia Department of Game and Inland Fisheries biologist who regularly gets cougar reports that don’t pan out, says he sometimes tells callers they probably saw a “U.F.O.—an Unidentified Furry Object.”

Some people just can’t be convinced that they didn’t see a mountain lion.

“They will not accept that what they saw is a bobcat,” says Patrick Tate, a wildlife biologist with the New Hampshire Fish and Game Department who says he has spent many hours chasing leads never to confirm one sighting.

“People get very upset with the department; they think the department is hiding that the species is here,” he says.

Though most tipsters are sincere, hoaxes run wild, say state officials. Mr. Tate says one photo making the rounds locally appeared to be a stuffed lion placed in the woods. The website of New York State’s Department of Environmental Conservation warns the public about “images of mountain lions circulating online with false claims they were taken in New York.”

“I don’t know why the public is so enamored with the idea of there being mountain lions at large in New York,” says Dan Rosenblatt, head of wildlife diversity at the department.

The state does believe that a cougar that wandered from South Dakota and was hit on a Connecticut highway in 2011 had traveled through New York, he says.

But typical accounts involve a fast-moving animal that appears much bigger than it is, says Mr. Rosenblatt, adding that biologists have used life-size cardboard cutouts of house cats and bobcats to help eyewitnesses sort out what they saw.

Hundreds of believers, meanwhile, have shared cougar stories on “New Hampshire Mountain Lion,” a blog dedicated to the topic.

There was the “absolutely massive beige cat” along Interstate 95 near Hampton, N.H., watching cars “as if waiting for traffic to die down so it could cross,” and the creature with a fat tail “crouched in a stalking position about 50 feet from my goat pen,” according to two accounts.

“It finally happened!!!!” Sargent Collier, a 74-year-old antique-firearms dealer who lives in a log cabin, wrote on the blog recently about the “big cat” that slinked across the road in Peterborough, N.H. “This ranks up there with the biggest thrills of my life,” he wrote.

The U.S. Fish and Wildlife Service invited public comments on its website through Aug. 17, and expects to rule on the Eastern cougar’s fate by next summer. Removing the subspecies from the endangered-species list would lift federal protections.

Some opponents say that action would be premature, and that Eastern cougars may just be skilled at eluding authorities.

“I am very clear on what I saw,” Thomas Cheney, a 56-year-old technology project manager, wrote the federal agency in July, saying he spied a cougar in Cary, N.C., and alerted the state only to “be condescendingly told I had seen someone’s pet.”

Linda McCracken, 67, of Marlow, N.H., wrote the agency “mountain lions are here…as much as officials don’t want them to be,” and complaining state authorities were wrongly attributing sightings to “even raccoons!”