The notoriously reclusive Calvin and Hobbes creator Bill Watterson – described, variously, as the "JD Salinger of the strips" and the "Bigfoot of cartooning" – has made a surprise venture out of an almost 20-year retirement to feature in the Pearls Before Swine comic strip.

Watterson's guest appearances see artist Stephan Pastis ceding control of his strip to a second-grade girl called Libby – a nod to Bill – after she mocks his drawing, and boasts that she can do it better. "Here's my pen, Lib … knock yourself out," says the cartoon version of Pastis. The strip then changes in style to feature a crocodile eating Pastis – "now we drawn gud, this strip funnier already," says an onlooking crocodile – a Martian robot attack, and Pastis surrounded by adoring women.

Watterson's panels appeared in the syndicated Pearls Before Swine strip last week, prompting fevered speculation from the legions of fans about an author who has done only a handful of interviews since he ended the much-loved Calvin and Hobbes strip in 1995, and rarely produces new work. "The lettering is a perfect match, it has to be him," wrote one fan on Friday. Another added that: "If it's not him, it's a damn good imitation."

Pastis eventually admitted to the collaboration this weekend, saying that "the only thing Bill ever asked of me was that I not reveal he had worked on Pearls until all three of his strips had run".

"And so I did not reveal his participation until now. And it was the hardest secret I've ever had to keep. Because I knew I had seen something rare. A glimpse of Bigfoot," he wrote on his blog.

Pastis had tried – unsuccessfully – to meet Watterson in the past. He described the Calvin and Hobbes creator as the "Bigfoot of cartooning", and as "legendary [and] reclusive", and said that he decided to make another attempt via email, after featuring a strip of Pearls Before Swine in which his newly-single cartoon self picks up a woman by pretending to be Watterson.

"And what do you know, he wrote back," said Pastis. "He had a great sense of humour about the strip I had done, and was very funny, and oh yeah … He had a comic strip idea he wanted to run by me. Now if you had asked me the odds of Bill Watterson ever saying that line to me, I'd say it had about the same likelihood as Jimi Hendrix telling me he had a new guitar riff. And yes, I'm aware Hendrix is dead."

Pastis told Watterson he would "do whatever you want, including setting my hair on fire", and Watterson wrote back, explaining his idea. "He said he knew that in my strip, I frequently make fun of my own art skills. And that he thought it would be funny to have me get hit on the head or something and suddenly be able to draw. Then he'd step in and draw my comic strip for a few days," said Pastis, who came up with the idea that "instead of having me get hit on the head, I would pretend that Pearls was being drawn by a precocious second-grader who thought my art was crap."

The collaboration began, and "Bill was funny and flexible and easy to work with", said Pastis. "At one point when I wanted to change a line of dialogue he wrote, I prefaced it by saying, 'I feel like a street urchin telling Michelangelo that David's hands are too big.' But he liked the change. And that alone was probably the greatest compliment I've ever received."

Watterson told the Washington Post that he'd first had the idea of appearing in the Pearls Before Swine strip "several years ago, when Stephan did one of his strips that mocked his own drawing ability and mentioned my strip in comparison".

"I thought it might be funny for me to ghost 'Pearls' sometime, just to flip it all on its head," he said. "It was just a silly idea, and I didn't know Stephan, so I never pursued it, and years went by."

But when Pastis got in touch with him earlier this year, Watterson realised the collaboration could be a way to raise money for Parkinson's research, "in honour of Richard Thompson", the Cul de Sac creator.

He is pleased with how the strips turned out, and the originals are set to be auctioned for charity. "It was generous of Stephan to let me hijack his creation, and more generous still to donate the originals," he told the Washington Post. "I had expected to just mess around with his characters while they did their usual things, but Stephan kept setting up these situations that required more challenging drawings … so I had to work a lot harder than I had planned to! It was a lot of fun."

Pastis referenced the celebrated ending of Calvin and Hobbes, in which the pair set off together on a sledge, Calvin saying: "It's a magical world, Hobbes, ol' buddy … Let's go exploring" in Saturday's Pearls Before Swine strip. He showed Libby – sledge in hand – telling his cartoon self that "I'm bored of drawing. Besides, there's a magical world out there to explore."

"But it's not even snowing," replies Pastis.

"Do I need to hit him over the head with the symbolism?" responds Libby, turning away.