The bunny was found dead at Chicago's O'Hare International Airport, the same place passenger David Dao was dragged off a United flight earlier this month
-- dragging United's reputation through the mud.
It happened when Simon, a 3-foot-long, 10-month-old Continental Giant rabbit, flew in from London's Heathrow Airport to O'Hare on April 19.
"I haven't got a clue who's to blame, but it's certainly very weird when Simon was so healthy," said Annette Edwards, a breeder who sold the bunny.
The bunny was on his way to a new owner and died in the cargo hold area while waiting for a connecting flight, Edwards told CNN.
"A man in America wanted him brought over. He wanted to bring him over as a pet," she said.
Simon is the progeny of Darius, who holds the Guinness world record for longest rabbit
. He potentially could have surpassed his father's size, Edwards said.
"I just feel that it's such a sad thing that I didn't really want to discuss it," she said. "It's never happened before, because if a rabbit has a full health check you don't expect them to die."
In a statement sent to CNN, United said it was "saddened to hear this news."
"The safety and well being of all the animals that travel with us is of the utmost importance to United Airlines and our PetSafe team," the statement said. "We have been in contact with our customer and have offered assistance. We are reviewing this matter."
The Department of Transportation reports on deaths on animals traveling by air every year and says the numbers are down.
According to its Air Travel Consumer Report (ATCR), 26 animals died in 2016. Compare that to how many animals were transported: 523,743.