The Parken Zoo in Eskiltuna near Stockholm put two of their own wildebeest on the menu for the event, the Daily Mail said.
Financial constraints were said to be behind the decision to cook the animals.
The wildebeest were put down prior to the event because the zoo could no longer afford to keep them, it has been claimed.
One of the workers at the zoo said he saw colleagues slaughter the animals and was surprised when they were used to feed staff.
"There was nothing wrong with the meat and we thought it was all right. But it feels a bit strange in retrospect," another worker was quoted as saying.
The zoo's chief executive Torbjorn Bergvall denied the meat was used for the barbecue and said somebody was trying to create bad publicity for Parken.
"These wildebeest were put down because they were aggressive towards other animals at the park. We did not have room for them all," the Daily Mail quoted him as saying.
The zoo has been in news before.
In 2012, it was accused of taking money from visitors, which was collected to save endangered species, and used it to pay wages.
It was also said to have put down several endangered species, including two cougars, to make way for more spectacular animals. It was later claimed the cougars died from natural causes, the daily said.