Last month, Lee Collings, 46, said he believed he had spotted a four-foot long crocodile in the nature reserve.
He said he had never seen anything similar in over 30 years as a wildlife photographer.
Lee claimed to have spotted the crocodile/alligator in the Fairburn Ings nature reserve.
He said: "I am 36 years old, and have been a birdwatcher for over 30 years, and I've never seen anything like this, other than a crocodile or alligator.
"It walked like it was rigid.
"It moved slow, but too fast to photograph it.
"By the time I realised what I thought it was. It was 3cto 4 ft in length, and about 4 to 5 inches high
"I'm not bothered about being ridiculed, I'm that confident about what I saw.
"The only thing that it could be, if there's any in the area is an Otter, but it was moving too slow and rigid."
Now, the RSPB said they 'they suspect that the sighting could have been of an otter' - which have been seen on the site.
The RSPB called for people to stay alert for wildlife as lockdown restrictions begin to be lifted.
The breeding season is now in full swing and wildlife is at its most vulnerable point of the year, the RSPB said.
Countryside birds, mammals and reptiles will normally avoid busier areas of human activity to ensure their nests and young are safe from accidental harm, steering clear of popular beaches, busy footpaths or dog walking hot spots.
The RSPB’s Director of England, Emma Marsh said “This year, nature hasn’t needed to adapt to human behaviour, as we stayed home, and some of our wildlife has reclaimed the places we’ve temporarily given up.
"As we head back out, we need to be alert and avoid disturbing nature as it gets on with producing the next generation.”