Archaeologists found the four-metre-long walrus with 1,500 human bodies as part of excavations at the station in 2003, amid the renovation of the station into the Eurostar terminal.
The bones were in a coffin with eight other sets of human remains, which are thought to have been used for medical research some time after 1822.
Both the archaeologist who discovered the bones, Phil Emery, and the zoologist who studied them for the Museum of London Archaeology, Alan Pipe, are not sure how the bones came to be in St Pancras Church on the northern side of the station.
“It’s a bit of a mystery”, Mr Emery told The Times.
“We did some research to see if we could find any record of a walrus being dealt with, for example, by the London Zoological Society, but we drew a blank.”
Mr Emery said there was a reference to Prince Albert “riding on the back of a giant tortoise”, but added that this was not relevant to the bones they found.
“It is possible the animal bones were being dissected as practice, but it does seem to be a rather exotic animal to be dealing with”, he added.
The most plausible reason for the walrus being in London was that it was brought to the city by whalers and sold for medical research or as a curiosity, they said.
While Mr Pipe has found the remains of an ostrich and big cats on archaeological excavations in London, this was his first encounter with a walrus.
“Although we have considerable evidence of whales in London, from porpoises up to blue whales, in all my experience we haven’t had any other bones of walruses except small fragments of skull which have been chipped to get out the tusks”, he told the newspaper.
He said that walrus skins were used for buffing metal and there was a trade in their tusks for ivory.
While how it came to be in St Pancras is a mystery even to experts, the remains can now be viewed by researchers at the London Archaeological Archive and Research Centre in Hackney, east London.