An erstwhile German horticultural magazine, Möllers Deutsche Garten-Zeitung (“Möller’s German Garden Newspaper”), was a monthly publication that contained gardening tips and science articles about the plants of the world. They celebrated the first April Fool's Day of the 20th century in style, by peppering their April 1900 issue with fake articles featuring fake plants and fake gardening techniques, most of which were fairly obvious to regular readers.
There was a “tree strawberry,” which towered over would-be snackers. There was a trumpet that could kill all insects in its vicinity with a single blast. And then there was Echinocereus dahliaeflorus, a cactus covered in sharp flowers. According to the text, the cactus was found in Madagascar, and its strange blooms perfectly solved a longstanding argument between members of the German Cactus Society and the German Dahlia Society.
Everyone in the office must have had a good laugh over the tall strawberry and the bug trumpet. But as the months went by, E. dahliaeflorus proved unmemorable even to its perpetrators. As a later cactus compendium explained, “The April Fool joke is so cleverly concealed that the editor deceived himself”—when compiling new discoveries at the end of the year, he “carefully indexed” that particular fake plant among the real ones.
That indexing placed the fictional cactus in the rarified category of April Fool jokes that left a lasting legacy. If you Google Echinocereus dahliaeflorus, you'll find a few references to the plant as an April Fool joke, but The Plant List just flags the name as "unresolved." Read more about Echinocereus dahliaeflorus at Atlas Obscura, and see more in their week-long series of articles on historic April Fool's Day pranks.