In the novelette "Water Babies" by Simon Brown, an Australian policewoman investigates a series of murders that are occurring cyclically (every 30-40 years or so). It turns out the perpetrator is a seal-like creature with the habits of a crocodile--it lunges out of the water to attack people onshore.
The text of the story, minus a few pages, can be found here:
http://books.google.com/books?id=My...#v=onepage&q=water babies simon brown&f=false
Although I don't think it's exactly described as such, the creature sounds a lot like the bunyip, a Aussie cryptid.
http://en.wikipedia.org/wiki/Bunyip
SPOILERS
At the end of the story, Walsh jumps into the Nepean River to rescue her creature-attacked teenage daughter, only to be attacked herself. Her partner shoots it repeatedly, literally blowing the top of its head off, and it disengages from Walsh and swims a short distance away, then apparently dies and sinks to the bottom.
(Although there's talk of a mysterious something only needing to feed every 30-40 years, it's apparently not a supernatural being. Good old fire and lead works every time.)
That means, once help arrives, the body of the whatever-it-is can be retrieved.
So what if the events of the story actually happened? In 2006, a carnivorous seal-like being with a habit of attacking humans is discovered in Australia after killing several people, attacking a group of students on a field trip, and being ventilated by a couple of Aussie cops.
Given the bunyip in the story appears to be a predatory animal, I'm putting this in post-1900 instead of ASB.