Eight-year-old Alejandro was travelling with his owners from Phoenix to New Jersey, where the couple is moving home to.
"When he landed here in MI, he was alive at 6:30 a.m., and then at 8:20, he wasn't moving and it just doesn't make any sense to me", the dog's owner, Michael Dellagrazie said in an interview with WDIV-TV, referring to his Pomeranian, Alejandro.
Dellegrazie's attorney, Evan Oshan, told Good Morning America that the plane had made a stop in Detroit and Alejandro was checked on.
Delta adds it is conducting a thorough review of the situation to find out why this may have occurred to ensure it doesn't happen again.
"When he landed here in MI, he was alive at 6.30am and then at 8.20am, he wasn't moving and it just doesn't make any sense to me", owner Michael Dellagrazie told WDIV-TV.
Delta are investigating what happened to the pup, while the owner's family have hired a lawyer who worked on another case with similar circumstances. 'The dog is not a pet.More news: World Cup friendly plus Neymar updates
Oshan says he's determined to get to the bottom of the incident, so no other family has to go through this pain again.
In a statement to Newsweek, Delta Airlines said they are working with Alejandro's family and are conducting a review of the situation.
Alejandro, a Pomeranian, was found dead during a layover in Detroit on Wednesday.
So far the cause of death has not been determined. In March, United Airlines came under fire when a flight attendant forced a family to keep their dog Tokio in an overhead compartment during a 10-hour flight. "We assume full responsibility for this tragedy and express our deepest condolences to the family and are committed to supporting them".
The changes will not affect pets in the cabin. A flight attendant told the owner to put him in the overhead bin rather than under the seat.
A day after that incident, a 10-year-old German shepherd named Irgo was flown to Japan when he was supposed to end up in Kansas.
In 2017, 18 animals died on United, three-fourths of all such deaths on US airlines.