UPDATE May 8, 1:25 p.m.:
Simon the 3-foot giant rabbit might have been mistakenly placed in a freezer by airline employees, which could have led to his demise, reports USA Today.
Simon was also cremated by United without the permission of the group that owned Simon, making an autopsy impossible. Des Moines trial attorney Guy Cook, who represents Simon's owner group is calling for an independent investigation into Simon's death, calling the cremation the destruction of evidence.
ORIGINAL STORY: United Airlines' passenger troubles apparently aren't limited to humans.
A rabbit that was likely to grow to become the world's biggest was founded dead after a United flight from London to Chicago, The Sun reported.
Simon, a 10-month-old continental giant rabbit that was already three feet long, is the offspring of Darius, currently the world's biggest rabbit. Breeder Annette Edwards said Simon was being transported to a "very famous" new owner.
Something very strange has happened and I want to know what. I've sent rabbits all around the world and nothing like this has happened before.
Edwards said Simon was healthy when he got on the plane and passed a veterinary check up just hours before the flight.
United said it is investigating the incident. Edwards said the airline was looking at CCTV images to see what might have happened to Simon.
United has been under fire after a video of David Dao being dragged off a flight went viral, prompting threats of a lawsuit. Just weeks before, the airline was criticized for denying children on board a plane because they were wearing leggings in violation of a dress code.
The Associated Press contributed to this report.