The creature is believed to have killed at least three fully-grown birds at the lake, leaving only a smattering of feathers as evidence of the crimes.
Witnesses have so far been unable to identify the perpetrator, although pike, catfish and even mink have been suggested as possible culprits.
Local councillors are now warning schoolchildren not to go paddling at the site, and dog owners have been being asked not to let smaller animals swim in the waters.
One dog walker described her horror at seeing a mallard disappear into the water at Stonebow Washlands in Loughborough, Leicestershire, never to be seen again.
She said: “I saw two mallards and the female was flapping her wings. I thought she may be cleaning herself, but she was quite frantic and was going up and down. Next thing I knew she was gone.
“I went over to have a closer look. The male was still there and I was about 30 feet away watching him intently. I stood there for two or three minutes and then in a flash all that was left on the water was a few feathers.”
The number of ducks on the lake have dwindled since the killer creature started terrorising the area.