Zoo director Ivan Ivanov says an unknown number of dogs leapt through a fence and attacked eight mouflon, four fallow deer and a doe.
Ivanov told The Associated Press on Saturday the incident occurred last week and was the most serious in the zoo's recent history. He believes freezing temperatures and hunger had driven the dogs.
Two fallow deer and one mouflon managed to escape and survived before security guards intervened. The dogs also escaped.
Ivanov said that new animals of the same species have been already transported to Sofia. The zoo is the largest in the country with 1,310 animals of 274 different species.