Ten-month old Simon was one big bunny. He was bred by a woman in the United Kingdom and sold to a "celebrity" in the United States.
Simon's breeder said he visited a vet hours before boarding a United Airlines flight from London to Chicago and was reportedly "fit at a fiddle."
After the plane landed at Chicago O'Hare International Airport, the bunny was fine, but apparently, he was found dead a short time later in a United Airlines holding facility at the airport.
United issued a statement saying, "We were 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. We have been in contact with our customer and have offered assistance. We are reviewing this matter."
Deaths on or after airline flights are not unusual.
There were 26 last year, according to a Department of Transportation report.
The most deaths in 2016 were on United, nine total.
In 2015, there were 35 deaths of animals on planes overall and 14 of those were on United, more than anyone else.
Examinations done by the airlines show most of the deaths were natural causes like heart failure and respiratory failure.
Some breeds of dogs, particularly those with short noses, have a hard time breathing in the air.
Those dogs often are at risk whenever they fly.
There's no word on how giant bunnies handle air travel, but the breeder has said she's shipped many of them by air without any issue.