Flight 3996 was carrying 33 passengers from Newark, New Jersey, to St. Louis, but the pet was due to fly from New Jersey to Akron.
Then, days later, the airline sent a German Shepard that was supposed to go to Kansas was sent to Japan.
United said that the dogs were put on the wrong planes when being moved from connecting flights in Denver.
The airline reportedly compensated the passengers on the diverted flight.
Ian Petchenic, a spokesman for the flight-data company Flightradar24, said the flight averages about two hours.
US helicopter crash in Iraq kills all aboard
The names of those killed - everyone on board - will be released after next of kin have been notified, the statement added. The US acknowledges that it has about 5,200 troops in Iraq that are part of a coalition fighting Islamic State militants.
The incident has brought upon a new policy for United Airlines. The airline came under intense scrutiny after a French bulldog puppy was forced into an overhead bin on a Houston-to-New York flight on Monday and died on the plane.
The Transportation Department is investigating the incident.
On Tuesday, a 10-year-old German Shepard named Irgo was flown to Japan instead of its intended destination in Kansas City, Missouri, the airline said.
Thankfully, Irgo's cross-Atlantic journey ended on Thursday and he is back home with his family.
Noonan declined to say if the company is creating or modifying regulations after the rash of dog-related mishaps.
United Airlines is retiring its fleet of Boeing 747 aircraft that are affectionately known as the ÒQueen of the SkiesÓ and marked the occassion with a final flight from San Francisco to Honolulu, Hawaii.