Even his byline was fake. He often went by the name Jimmy Rustling, as he did on this story, in which he claimed protesters were getting paid $3,500 to disrupt Trump rallies.

That article, which was tweeted out by Mr. Trump’s campaign manager, now has a taunting disclaimer at the top that says the story isn’t real: “I personally went to two Donald Trump rallies and I can say with 100% certainty that NONE of the protesters were getting paid,” the statement says. “This story I wrote is mocking all of you sheep who think protesters are getting paid.”

His domains, like newsexaminer.net and cnn.com.de, had names conveying a whiff of legitimacy.

On the former, a story surfaced appearing to be an interview with Mr. Horner. He was described as a former Secret Service agent who had written a book to expose “the real truth” about Barack Obama. The fake story said that Mr. Horner was “one-hundred-percent positive that President Obama is not only gay, but a radical Muslim as well.”

It was so widely shared that The Associated Press debunked it last year.

In an interview with The Washington Post in November, Mr. Horner said he was the reason Mr. Trump had been elected. The interview led to a round of media appearances.

“My sites were picked up by Trump supporters all the time,” he told The Post. “I think Trump is in the White House because of me. His followers don’t fact-check anything — they’ll post everything, believe anything. His campaign manager posted my story about a protester getting paid $3,500 as fact. Like, I made that up. I posted a fake ad on Craigslist.”