Here's why they fit your description:
* Size: They can grow to the size of small dogs, even reaching up to 20 pounds.
* Beaver-like: They have a similar body shape to beavers, with a stout build and large, rodent-like teeth.
* Rat tail: Their tails are long, thick, and hairless, much like a rat's tail.
* Long whiskers: They have long, prominent whiskers.
Important to note: Nutria are invasive species, introduced to many parts of the world, including Europe. They are originally from South America. They are not native to Italy but are now found in several regions, including near Florence.
If you are unsure: It's always best to err on the side of caution with wildlife identification. If you're unsure if what you saw was a Nutria, you can try reaching out to local wildlife authorities or experts in the region. They will be able to give you more accurate information.