What do these creatures have in common: auroch, bear, beaver, boar, lynx and wolf? Answer: they are all mammals that became extinct in Britain. It is a list to bring out the romantic in us all: imagine a Britain in which aurochs lurked in their bellowing herds, where bears waited for you to go down to the woods today, where beavers went in for major construction projects, where wild swine rootled and snuffled, where the lynx led their secret predatory lives and where wolves howled to the Moon on clear-skied, frost-bitten nights.

Some of us do more than imagine. It’s too late for the auroch, the wild ancestor of Europe’s domestic cattle, which is extinct worldwide. But wild boar have come back, escaping or being released from farms and returning to the forests.

The chances of bears coming back are remote, for all that they live cheek by jowl with civilisation in North America, but there is frequent talk of reintroducing wolves to one of the Scottish islands. There are also some who insist that the lynx could be a perfectly viable British animal again. Which leaves us with beavers.

Beavers are not only back, they have built their first lodge. Beavers don’t only build dams: they also make extravagant dwellings. Now, in Knapdale Forest in Mid Argyll, and for the first time in 400 years, there is a lodge built by British beavers.

It’s a pretty hefty thing. For all that, you or I would walk straight past it, seeing it as merely a pile of sticks and mud and leaves, unless you had an eye for beavers and their works. It stands 2m high and covers an area 5m by 7m: pretty elaborate stuff for a creature described as “the size of a chubby spaniel”.

A classic lodge is a two-room structure, with an underwater entrance. There is a wet room and a dry room: a beaver enters the wetroom first and shakes itself dry before moving into the cosier inner room. And one such structure has been built in the forest only a few months after the beavers returned to this country.

Three beaver families were reintroduced into Knapdale Forest in May. They were all wild-caught Norwegians. This project, which involves the Scottish Wildlife Trust and the Royal Zoological Society of Scotland, has required elaborate labour to make it possible. The hardest job was convincing landowners that the country would not be overrun by beavers and dams and other fancy Norwegian affectations, demolishing trees and destroying forests. And, no, they won’t eat your salmon, because they are vegetarians.

All the concerns were soothed, eventually, and the beavers made it to Scotland. Not that it has all been plain sailing. One family was broken apart by illegal shooting from some trigger-happy maniac. After this, the female disappeared, the male set off after her and the remaining young female also left the original site. Elsewhere, a young male died.

These things are not easy. That is why the construction of this gigantic lodge is a matter of significance and celebration. Beavers acting as wild beavers should, and in a place where they have not been seen for centuries: all thanks to a bunch of hard-working, hard-talking visionaries. I saw my first beaver last year: a Canadian, and therefore technically a different species, but similar enough. I was in British Columbia looking for bears, but a stroll to a known beaver-pool produced two beavers as if by prior arrangement — looking for wild mammals isn’t supposed to be that easy.

I was struck by their size, about twice as big as I had imagined — a very chubby spaniel, indeed — and by their swaggering waddle. These animals were used to visitors, so they paid me no mind whatsoever, coming within five yards or so, dunking themselves in the pool and snacking on salad. Big, bumbling, benign and one of the few mammals given to major modification of the environment, beavers also help to open up the forest and bring opportunities for other water-loving beasts thanks to their practice of building dams and creating deep-water pools. Many of our forests are in an unnatural state because they lack the beavers to modify them.

The early months of the Scottish beaver trial — these are fully devolved rodents — have been a cautious success, ending with the triumph of the lodge. Every move has been monitored independently and the bout of construction has pointed the way forward. Early days, early days: but the beavers are there, and the future is in their paws. Time for romantics and practical conservationists to start thinking lynx.