An Irishman used forged documents to sell horns from endangered black rhinos to a New York collector for 50,000 dollars (37,000 euro), prosecutors said.
A judge ordered that Slattery be held without bail during an appearance in federal court in Brooklyn.
A criminal complaint alleges that in 2010 Slattery travelled from London to Houston to try to buy two horns at a taxidermy auction house.
Learning that he needed to be a resident of Texas to make the purchase, he recruited someone to do it on his behalf, known as a "straw buyer", the complaint says.
Slattery and unidentified suspects gave the straw buyer 18,000 dollars (13,300 euro) in 100 dollar (74 euro) bills to complete the deal, the complaint says.
Later that year, Slattery met a Chinese buyer in Queens and sold four horns using endangered-species bills of sale with fake Fish and Wildlife Service logos on them, the complaint says.
It is unclear where he got the additional two horns, it says.
Three of the five species of rhinoceros in Africa and South Asia have been hunted to the verge of extinction because their horns command exceptionally high prices for use in traditional Asian medicine chiefly in China and Vietnam, where the powdered horn is marketed as an aphrodisiac and even as a cure for cancer.
The horns are made of keratin, a fibrous protein that is the building block for skin and hair, and has no documented medicinal value.
In 2011, Europol issued a warning that an Irish criminal network based in the Co Limerick village of Rathkeale was responsible for dozens of thefts of rhino horns across Europe.
Europol said the thieves, called the Rathkeale Rovers, had already targeted museums, galleries, zoos, auction houses, antique dealers and private collections in Britain, continental Europe, the US and South America.
Earlier this year, masked men stole stuffed rhinoceros heads containing eight valuable horns from the warehouse of Ireland's National Museum in a heist being linked to the gang.