A child's depiction of the ninki-nanka on the expedition's blog

Believed to live in swamps, the ninki-nanka appears in the folklore of many parts of West Africa.

It is described as having a horse-like face, a long body with mirror-like scales and a crest of skin on its head.

Team leader Richard Freeman told the BBC, evidence so far was sketchy as most people died soon after seeing it.

Mr Freeman, a cryptozoologist from the UK-based Centre for Fortean Zoology, admitted that the ninki-nanka's existence was "very far-fetched indeed".

There seems to be this thing when you see the ninki-nanka you will die usually within a few weeks

Richard Freeman

Second-hand accounts varied wildly from it looking like a crocodile or a snake to having wings and spitting fire, he said.

But he disputed a suggestion that the hunt was a waste of time and money.

"We didn't know any of this before we came. We have to look into everything to see if there is a possibility that there's a real creature there," he told the BBC's Focus on Africa programme.

Cryptozoology is the search for animals whose existence is disputed or unsubstantiated, such as the Loch Ness monster.

Herbal potion

The team have interviewed one eyewitness so far - a park ranger from the Kiang West National Park who lived to tell the tale of his encounter three years ago.

One of the expedition team examines a 'scale'

He described an immense animal 50 metres long by one metre wide that he watched for more than an hour before being taken ill.

He put down his survival down to a herbal potion given to him by an Islamic holy man, Mr Freeman said.

Later, according to the expedition's blog, after being shown pictures of various reptiles and mythical animals, the ranger said the creature's face most resembled that of a Chinese dragon.

"We've heard very similar stories all over The Gambia but mostly not first hand eyewitnesses... there seems to be this thing when you see the ninki-nanka you will die usually within a few weeks," Mr Freeman said.

The team are taking back a sample of what is claimed to be a ninki-nanka's scale to be tested in the UK.

But initially investigations suggestion this is a red herring, perhaps a bit of rotten celluloid film and "not biological".

"We haven't discounted the possibility that there is a flesh and blood ninki-nanka in the swamps of West Africa, it's just at the moment the evidence is pointing to something more folkloric," he said.