Alien Cats and Black Dogs
Another part of the answer may lie in the local culture and folklore. Historically, England is known for sightings of mysterious black cats (sometimes referred to as Alien Black Cats, or ABCs) sighted prowling the moors and countryside.
Some were merely mistaken pussycats; in a few cases lynxes were found or captured; but in most cases the furtive feline disappeared without leaving any hard evidence of its identity.
The knowledge that large unknown cats are said to lurk in the countryside might prompt a person to jump to that conclusion if they see some strange feline animal, in the same way that visitors to Loch Ness may interpret anything odd in the water as Nessie.
And it's not just cats. Great Britain is also home to many centuries-old legends about ghostly Black Dogs as well, traditionally said to be evil omens. These stories likely inspired Conan Doyle's Sherlock Holmes novel "The Hound of the Baskervilles."
While Essex police were reluctant to suggest that eyewitnesses were hoaxing or exaggerating, they stated that following an extensive search of the area using helicopters, mounted police, infrared cameras and foot searches, they found no evidence of a lion: