Madeleine Sweet took the mastiff's picture Monday and said the dog flew first class.
"(My) initial tweet was about how big the dog was. It wasn't until I boarded, I could hear whispers of 'Oh my God, the dog is in first class,'" Sweet said.
While Sweet was waiting to board, a woman traveling with the dog approached flight attendants who were in charge of the boarding process. The dog's owner ended up sitting in the front row of first class, with the dog lying on the floor in front of her, Sweet said.
"It was huge. I have never in my life seen a dog that fat – it was massive," Sweet said.
But now the owner of the dog is speaking out, saying her 165-pound mastiff, Hank, is actually a service animal who detects her seizures.
Los Angeles resident Kari Whitman told NBC4 her 3-year-old pup was wheeled in on a dolly because he recently had surgery and can no longer walk. Whitman attributes part of Hank's large size to his inability to be independently mobile.
Regardless, Whitman said she loves Hank and he keeps her safe, so he must travel with her.