On the train from Connecticut to New York, passengers stared. One businessman looked at the dog and said, “ferocious.”

It turned out Marnie had 14 rotten teeth, which did not explain the dog’s most curious feature: her long Gene Simmons-like tongue that lolled out to the side, making it look as if she was always laughing. Marnie just had a very long tongue, Ms. Braha said.

Once she was cleaned and had dental surgery, Marnie was like a new dog.

She insisted on going everywhere with Ms. Braha and barked for hours when she was left alone. Marnie accompanied Ms. Braha to her favorite bar in Williamsburg in Brooklyn, and to parties and restaurants. A couple of times, Ms. Braha sneaked Marnie in to her job at MTV, where she was a producer.

“She loved to party,” Ms. Braha said. “You’d bring her out to a group of people, her tail would wag, her tongue would hang out, and she would run around in circles.”

In 2014, after the MTV division she worked for folded and she was laid off from her job, Ms. Braha began posting more and more photos of Marnie on Instagram.

It began as more of a hobby and a way to spend time with her attention-hungry dog than an effort to join the ranks of other pets who had become famous online.

“I was just doubtful anyone would care,” Ms. Braha said of her early pictures. “There is so much content online and it’s almost like the aspiration of fame seems so trite that it just didn’t appeal to me.”