The Guardian (with video):
The couple, who want to remain anonymous to protect the animal’s location, said: “Some of our trees began to go missing overnight and others were simply being mauled. It looked like someone was hitting them with a machete.
...
The discovery has baffled wildlife experts because Wales’s wild beaver population is virtually nonexistent. A small number in colonies several miles away are all accounted for.
(The suspicion is it's vigilante rewilding.)