Have you ever heard someone tell you about a rat they saw that was as big as a cat? The largest rat that lives wild in the UK is the Brown rat aka: Norwegian rat, Sewer rat, Common rat, Dirty rat (or was that James Cagney?) and it is not supposed to grow much bigger than 1lb/500g.
If a rat was the size of a cat it would have to weigh 5 maybe 7kgs, so that would make it at least 10 times the “normal size”!! This is highly improbable. Imagine it the other way round, a healthy adult domestic cat weighs about 11lb/4-5kg and the heaviest living cat is thought to be a 39lb tabby in Albuquerque USA www.worldrecordsacademy.org, if such “rat to cat” super-sizing were to happen it would give us a 100lb cat!
So what could people be seeing?
Probably a Coypu – it is a South american rodent that lives in and around rivers and does grow to the size of a cat! They are to be seen in Southern England, escapees from fur farming or pet owners.