Since then, plenty has changed in Philadelphia. The club has two World Series titles, a new home at Citizens Bank Park and powder blue is long gone from its color scheme.
But some things never change, and since 1978 the Phanatic has been one of them.
"Even though it was new, it really took off right from the beginning," said the man who portrays the Phanatic, Tom Burgoyne. "The fans kind of took to the Phanatic, and I think you could say he's evolved. But he's really the same personality and the same character as he's been for the last 30, 40 years."
The Phanatic turned 34 on Wednesday, and the team will celebrate his birthday on Sunday by inviting seven other baseball mascots to the game and presenting him with a card signed by fans and a birthday cake.
The club has celebrated his birthday in different ways over the years -- including a magic show, a circus and a roller derby. Being an Olympic year, this year's festivities will include several races for a gold medal -- a potato-sack race, a three-legged race, a wheelbarrow race, a tricycle race and an obstacle course. The mascots will be competing against young fans.
For Burgoyne, one of the best aspects of the Phanatic's character is the way he has constantly interacted with the fans in Philadelphia, and the way those fans have embraced the Phanatic in return.
"The Phanatic's been there every game," Burgoyne said. "He's always there, and its consistent. He's the same character. People know that personality, it's almost like a broadcaster that's been around for a long time and really becomes part of the family. That familiarity certainly helps."
Publicly, Burgoyne refers to himself as the "best friend of the Phanatic" to keep up the playful notion that the Phanatic is a real creature. He began portraying the Phanatic as the backup to original portrayer David Raymond in 1989, taking over full time after the 1993 season.
A Drexel-grad and lifelong Philadelphian, Burgoyne has been a Phanatic fan since its inception in 1978. He got the job after responding to a newspaper ad in the Philadelphia Inquirer.
"I answered the ad, and 25 years later, here I am doing what I'm doing," Burgoyne said. "I got lucky."
These days, in his furry green costume, Burgoyne rides around on his ATV before the game and is constantly messing with opposing players and umpires. He stands on the rooves of dugouts, performing his classic dances. He'll be joined on Sunday by a cast of mascots that will include the Pirate Parrot, the Mariner Moose and Wally the Green Monster.
Among those other mascots, the Phanatic has seemed to stand out -- withstanding the test of time. Burgoyne has a theory as to why.
"The Phillies have always done it the right way, too," Burgoyne said. "He's always gotten away with some things on the field, goofing around with the players before the game. That helps. Maybe some other mascots aren't being highlighted or are not allowed to go to certain areas of the ballpark."
Burgoyne said he is careful not to interfere with play, and he makes sure not to distract opposing players or umpires to a point where they might lose focus or look bad.
But that said, the Phanatic always seems to be testing the limits, like a mischievous child looking to ruffle whatever feathers he can.
"He definitely is a kid at heart," Burgoyne said. "He's got that personality of a 10-year-old kid that gets into trouble every once in a while. But you can't help but love him."
AJ Cassavell is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.