Several Mets stars have intimidating superhero nicknames: Noah Syndergaard is Thor. Matt Harvey is the Dark Knight of Gotham. David Wright is Captain America.

Others have cool monikers based on their names: Jacob deGrom is deGrominator. Curtis Granderson is the Grandy Man. Yoenis Cespedes is known simply as Yo.

Then there’s Michael Conforto. His teammates call him Scooter.

“I don’t know why,” he said. “Whatever the guys want to call me, it’s fine with me.”

Scooter, the 23-year-old outfielder in his second season with the team, says he has had several nicknames in his life. He has been called Hammer because of his initials, MC, a reference to 1990s-era hip-hop artist MC Hammer. He’s also been called Toe and Forts, from shortened versions of his last name. Granderson just calls him Forto.

But pretty much as soon as he was called up to the majors last year, somebody dubbed him Scooter. And it stuck.

“I just call him it because when I got up here everyone else was calling him it,” said rookie starter Steven Matz.

Figuring out how it started is harder than it sounds. The first people to call him Scooter were members of the Mets’ pitching staff. But even some of them, like Matz, are baffled by its meaning. That hasn’t stopped them from calling him Scooter, of course. It’s the name they use to keep track of the basketball rankings for the mini hoop they have in the clubhouse, or when they’re simply trying to get his attention.

“I just kind of ran with it because I thought it was really funny. I wish I could tell you,” Syndergaard said. “It kind of rolls off the tongue. He looks like a Scooter!”

“ I wish I could tell you. It kind of rolls off the tongue. He looks like a Scooter! ” — Noah Syndergaard

Why it’s funny or what type of Scooter he looks like is less clear. The handle has deep roots in both popular culture and baseball history, but an investigation into the moniker revealed that even those were dead ends. Like so many kids, Scooter had a scooter when he was younger, but said, “I don’t think that has any relevance.”

Then there’s the “The Muppet Show,” the iconic puppet program starring Kermit the Frog and Miss Piggy. Scooter was an orange stagehand on the show with a shock of hair and eyeball spectacles. Ryan Gennett, the Milwaukee Brewers second baseman whom fans know as Scooter Gennett, has said that the Muppet was the inspiration for his nickname as a kid.

J.D., the character played by Zach Braff on the television show “Scrubs,” is occasionally referred to as Scooter and has a motorized scooter he calls Sasha. And on “SpongeBob SquarePants,” there’s a parrotfish named Scooter. But none of these seem to be the inspiration for the Mets’ Scooter.

More notably, the nickname has a history in New York baseball: Beloved former Yankee Phil Rizzuto was known as Scooter from his playing days as a Hall of Fame shortstop through his career as a commentator. But it was widely known that Rizzuto’s nickname stemmed from his diminutive 5-foot-6-inch frame and the quickness with which he carried it around the diamond.

Michael Conforto can’t figure out why everyone calls him Scooter. Photo: Elsa/Getty Images

Scooter Conforto, on the other hand, is 6-foot-1 and muscled, and he was 3 years old when Rizzuto retired from broadcasting. Syndergaard, also 23, didn’t even know who Rizzuto was, apart from the reference to him in the Adam Sandler movie “Billy Madison,” much less his nickname.

All of this has made the Scooter story a mystery, even to Scooter himself. “They don’t remember who started it, but it happened,” he said.

It’s even stranger in a locker room with stars whose nicknames have become so recognizable among Mets fans. They wear Batman masks to Harvey’s starts and carry hammers when Syndergaard is on the mound. Syndergaard even walked around Manhattan in a Thor costume, complete with helmet and cape, earlier this season. Matz says he’s never had a nickname that has stuck, though that’s perhaps more understandable than an inexplicable nickname.

While Scooter has been mired in a slump lately—he went 0-for-6 in Wednesday’s 2-1 extra innings loss to the White Sox and is 1-for-22 in his last six games—there’s little doubt that he’s part of the Mets’ crew of rising stars. Despite the rough patch, his .813 on-base-plus-slugging percentage ranks third on the team and eighth in the majors among players 23 or younger.

That he performed so well so quickly (he had an .841 OPS in 2015 and hit three home runs in the playoffs) came as a bit of a shock after he was selected 10th overall in the 2014 draft. He was the second-fastest position player from that draft to reach the big leagues.

So maybe the nickname was a bit rushed—and if Scooter doesn’t know Scooter’s meaning, it’s possible that nobody does.

In the meantime, Matz is sticking with his explanation: “It means a good left fielder. That’s what it means to me.”

Write to Andrew Beaton at andrew.beaton@wsj.com