In Yona 3:7-8 we find that the people of Ninveh included the animals in their teshuva process. Why make the animals fast and don sackcloth? How was this part of the teshuva process?
It may refer to the people who are like animals because they don't recognize Who created them (see Rashi to 4:11).
The animals may have been involved in sin such as gilui arayot and avodah zara so although they must have been induced in doing sin, and were probabaly surrounded by sin, they too were made to repent. Their owners who helped commit the sins repented and made their animals fast too. BTW: It is interesting that Hashem argues with Yona about having pity on the people and the animals.