I must once more reiterate that I do not subscribe to any of the following alternative theories though if any of them ever proved to be validated I do think life would be far more interesting that it is !!
Animal Mutilation Theory: Simply states that manyu of the carses of dead wildlife and inparticular livestock in the UK and throughout the world is being undertaken by Aliens for as yet know unsnkown reason.
Ghost Cats: That sightings of big cats are ghosts and the 'proof' of this is that as no conclusive evidence has been found to date of big cats existance in the UK then they have to be ghost cats.
Interdimentional Cats: Please see ghost cats above.