Beavers are busy indeed, but not eating fish: they are strict vegetarians. Their tails are good for many things, but not propulsion. Beavers do not harm fish. You’re thinking of an otter ripping into a struggling trout.
Giant tortoises are a recent evolutionary innovation and used to be everywhere, not just on islands. It’s impossible to age them accurately unless you know when they hatched. They are actually pretty slow. The reason why the tortoises Darwin saw in Galapagos moved so fast was probably because he was chasing them and they were running away.
Lonesome George died 10 years ago. Now there is a new contender – a tortoise called Fernanda – for the most lonesome creature on earth.