All artists are by nature squirrels, hoarding away the kernels of creative ideas to feed their inspiration through fallow months. Preliminary sketches too precious to part with are stuffed into portfolios and stashed away in nooks and crannies for future reference. Rarely sold during the artist’s lifetime, these priceless scraps often emerge into the light only after – sometimes long after – the artist’s death. The record is probably held by Welsh 18th century painter Thomas Jones, whose now famous oil sketches of Italy only came to light 150 years after his death.

In the case of Welsh artist by adoption William Brown, it has happened sooner. A spring-clean of the marital bedroom in Bridgend by his widow Carys recently uncovered, hidden under the bed, a cache of hitherto unseen sketches in charcoal and poster paint – glimmerings of ideas for future paintings when they were still a twinkle in the artist’s eye.

Brown probably ranks as one of art history’s most prolific sketchers. When in full flow, said his London dealer David Solomon, pages fell from his hands like autumn leaves. “It’s my thinking process,” was the artist’s own explanation, “I draw like people knit. I can sit at a table and do 200.” The difficulty was “knowing when to stop, knowing which ones” – the drawings under the bed were the ones he stopped at.

On 10 October some 30 examples, as fresh as the day they were drawn, will go on show with related paintings at The Art Shop, Abergavenny in an exhibition titled From Under the Bed. The title is appropriate in more ways than one, and has a particular resonance for me. What first brought the unique genius of William Brown to my attention as the then editor of Artists & Illustrators magazine was a 1996 Glynn Vivian Gallery exhibition similarly titled What’s Behind the Blanket? And blow me down if the first version of its title painting hasn’t come out from under the bed.

It was that touring exhibition that introduced Brown’s colourful cast of animal characters – none more colourful than the artist himself – to a wider audience, an audience that had never seen anything quite like it. Blinking into the art world spotlight from behind the blanket came the French Loup Garou, the goggle-eyed werewolf of the Toronto-born artist’s Canadian childhood, and troupes of what would become his trademark bears in their distinctive yellow and red-checked livery – the McLure tartan of his Ayrshire grandmother. Drawn with the vigour and dash of a Picasso and painted in colours to make Matisse pale, the visual onslaught of his imagery was unstoppable. As soon as the catalogue landed on my desk, I rang the gallery to ask for an interview.

It didn’t happen for two years, during which time memories of an earlier trip to Libya had added exotic new props and backdrops to the repertoire. The forest-dwelling werewolf of the frozen north was transplanted to the unfamiliar holiday location of broad prickly pear-fringed bays and sparkling seas, their cerulean blue expanses broken only by the faintly sinister presence of a plane or tanker – Brown’s visit had coincided with the US air strikes on Tripoli. Sketches of a tanker, planes and a Loup Garou among palm trees – dressed in holiday pink – have now emerged from under the bed.

A restless traveller, Brown never returned from a foreign adventure without a local folk myth or two packed into his mental baggage. But it was South Wales, where he settled in 1991 after his marriage with Carys, that produced the richest seam of stories, and a new star turn in his animal circus: the Mari Lwyd. In Brown’s imagination the skeletal ghost horse of South Walian legend acquired the melancholy and mildly comic aspect of a revenant Eeyore crossed with a two-legged pantomime horse. Welsh love spoons came later, and the Venus of Blaengwynfi, dreamed up as a Celtic sister to the Venus of Willendorf. Meanwhile the expatriate Brown bears settled down and dozed under cottage windows opening on watery Welsh moons, and the pine-clad expanses of Canada gave way to the small round hills of Glamorgan, typically pictured – as in the painting Cwmafon in this exhibition – with a row of treacherously leaning miners’ cottages clinging together for support along the bottom.

In 1997, a collaboration with French poet Lucien Suel on Le Nouveau Bestiaire sparked a vivid series of animal woodcuts, a print medium at which Brown was a natural – dispensing with the clobber of a press, he hand-burnished prints with the back of a wooden spoon. Never cut out to be a mere illustrator, he protested that he would “rather work with the poets – unless they’re dead, which is inconvenient.” The posthumous fruits of his final collaboration with Welsh poet David Greenslade on a hand-printed book of woodcuts, The Dark Fairground, will be unveiled on the opening night of The Art Shop show.

It might seem curious that a born colourist like Brown could express himself so vividly in black and white, but his monochrome was other people’s colour. In a black and white painting in the show, The Sky at Night, traces of grey-green underpainting creep around the image and warm it into life: “I surprise myself,’ he once confessed, “I am capable of subtlety.” He was also capable of surprising seriousness. The recurring theme of The Dark Fairground is a nagging reminder of the shadow world we prefer to forget. The bears in Passing Through II file in and out of an area of shadow, slipping from chiaro into scuro like circus performers swallowed by the darkness behind the tent-flap.

What is the meaning of it all? When put on the spot, the artist’s own instinct was to dive behind the blanket and leave his audience scratching their heads. The design of his pictures might be glaringly simple, the colour dazzlingly pure and the calligraphy – squiggles for waves, zigzags for pine trees – instantly readable by a child, but the meanings remain tantalisingly elusive. Writing his obituary for The Independent, I realised from the lacunae in his life story that there was a lot behind the blanket of his own biography that had been deliberately kept from the light. The man who painted the disarming sketches in this show of the Trojan Horse as a bright orange toy with outsize wheels was something of a dark horse himself. But that, of course, is what gave his work its edge.

Opening up the new batch of sketches on my desktop, several images made me laugh out loud. It was laughter of pure pleasure, but also surprise that the creatures of Brown’s artistic imagination could spring so disconcertingly into independent life. Rereading the notes of our first interview, I was astonished to find that he had told me with uncanny prescience: “I want to do all the bogeymen in the world. What’s behind the blanket leads to what’s under the bed.” What else may lurk behind the sofa and under the dresser? We haven’t seen the last of William Brown.

Laura Gascoigne, art critic.