The creature was spotted under the Brooklyn Bridge by Denise Ginley, who sent photos of it to New York-based news website Gothamist on July 22.
Ms Ginley said the animal was about 60cm long from head to tail and asked: "Is this another incarnation of the Montauk Monster, or just the biggest rat in the city?"
Photos of the carcass, which Ms Ginley dubbed the "East River Monster", quickly spread throughout US media, with people suggesting it was a washed up rat until the New York City Parks Department claimed it was a pig.
But this was quickly disputed, with people pointing out the photos showed the creature had claws and not trotters like a pig.
Ms Ginley told ninemsn she was walking to a farmers’ market with her boyfriend when she first spotted the carcass and she returned later that day with her camera to photograph it.
"Several other people were looking and taking photos, and everyone had a different opinion about what it was (suggestions included giant rat, dog, pig, monster, and seal)," Ms Ginley said.
"Obviously I thought it was gross, but it was just so strange looking I wanted to get closer … strangely enough, it did not smell, maybe because it had been in the water so long."
The carcass has still not been identified, and Gawker jokingly reported that the New York City Parks Department was "still being cagey" about the animal.
Another unidentifiable animal, the Montauk Monster made headlines in July 2008 when it washed up on a beach in Montauk, northeast of New York.
At the time people suggested the creature was a raccoon, a small dog, a rat and even a bloated turtle without its shell.