Federal agents peered into a duffel bag on the Mexico border. They found a tiger cub.

The three men crossed the U.S. southern border into Texas with a black duffel bag, on an apparent mission to deliver their lucrative product to the United States.

But they caught wind of border agents nearby, readying to intercept them near Brownsville, officials said.

It led to a calculation: Now what? And what to do with the unconscious tiger cub weighing down the duffel bag?

The men retreated back into Mexico, leaving the bag, and the male cub became an unexpected ward of U.S. Customs and Border Protection.

“NOT an average day in the field,” Irma Chapa, a spokeswoman for the Rio Grande Valley sector of CBP, said in a tweet Monday.

The tiger was turned over Brownsville’s Gladys Porter Zoo. It appears to be two months old and in good health, but it is likely that the cub was tranquilized to keep him still and quiet during the journey, senior veterinarian Tom deMaar told The Washington Post.

“You’re not going to stuff a live tiger into a bag without an argument,” deMaar said Tuesday.

The tiger has not been given a name, deMaar said. But the cub was recovered on the day Mexico celebrated Dia del Niño — or Children’s Day, so some at the zoo have given him an unofficial moniker, the veterinarian said: Niño.

The cub was an apparent node in the billion-dollar trade of animal smuggling, and it is in unfortunate company.

Last year, a tiger cub bought by a U.S. teenager was intercepted at a checkpoint in California.

In 2010, border agents recovered a caged and abandoned tiger in Laredo, another Texas border town.

And a decade ago, six tiger cubs were recovered from an attempted sale at a Walmart parking lot, also in Brownsville. They appeared to be southbound for Mexico, local media reported at the time.

deMaar said those six cubs were also delivered to Gladys Porter Zoo — an inadvertent collection point for tigers caught in an international smuggling ring. Three are still at the zoo, he said, and two others were sent to a sanctuary. The sixth tiger died.

About 350 million plants and animals are sold around the world annually, generating between $7 billion and $23 billion, Washington-based conservation group Defenders of Wildlife said in a 2015 report.

Latin America has emerged as a fulcrum in the endangered and exotic species trade feeding the booming U.S. market, the group’s senior international counsel Alejandra Goyenechea told The Post.

A quarter of the 50,000 animals and wildlife products seized at U.S. ports of entry from 2005 to 2014 originated in Latin America, the group found in the report, which Goyenechea co-authored.

Smugglers move freely through corruption-plagued nations and exploit the sometimes-porous border with Mexico to send endangered animals and illegal animal products into the United States. And they tend to use trade routes already traversed by drug, weapon and human smugglers, a highway of sorts of illegal activity.

“Why establish a new route?” Goyenechea said.

But two additional circumstances make the region a perfect storm for exotic animal poaching, she said.

Brazil, Mexico and Colombia are among the most biodiverse nations in the world, providing an unbroken land bridge to traffic exotic animals from those fragile ecosystems to the United States.

And when they arrive, smugglers can sometimes outmaneuver federal agents. Out of 328 ports of entry in the United States, 64 are staffed by Fish and Wildlife Service officials. And of those, only 18 sites have wildlife-focused agents full time, Goyenechea said.

Nearly 55,000 animals were seized at ports of entry from 2005 to 2014, with an unknown number that arrived in the United States undetected, she said.

Queen conch shells housing snails, prized by seafood chefs but subject to fishing bans in Florida, are the most commonly smuggled species from Latin America, Defenders of Wildlife found.

Sea turtles and iguanas are also trafficked in large numbers, with caiman and crocodiles smuggled and skinned for boot and belt leather.

“There’s a demand for exotic skin boots, and the more exotic, the more people want them,” Nicholas Chavez, the southwest region special agent for the U.S. Fish and Wildlife Service, told National Geographic last year.

And Mexican parrots are endangered by the demand for protected and rare birds, like the vividly colored orange-fronted and yellow-headed species.

Many smugglers rely on methods less conspicuous than black duffel bags carried by hand. Last March, federal agents intercepted a shipment of cobras hidden in potato chip cans.

While methods of stuffing animals into containers and luggage may be helpful to smugglers, the confinement is dangerous, Goyenechea said. Many animals die in transit from suffocation or hunger, and others are injured.

That is not lost on smugglers. To sell a single animal, traders will collect some species at a factor of 3, 5 or even 10 because some of the animals are expected to perish along the way, she said.

“It’s an additional threat to wildlife,” Goyenechea said.