It is something lots of people, including those who I have met in real life, claim to have seen. And it's something you often hear from random people eg https://www.youtube.com/watch?v=9FGlJoK1A_A
A thread popped up on /out/ before, and several anons claimed to have seen them in the UK (specifically one in cannock chase) and it has been something which has appeared as a 'modern legend' alot (beast of bodmin moor etc)
I was thinking about how mysterious large cat/dogs are a feature of ancient english folklore, too, such as the shuck.
Anyway just wanted to see some people's ideas on what it could actually be or if its just some kind of mass delusion of people seeing feral house cats.