In my opinion, however, it is certain wicked demons, and, so to speak, of the race of Titans or Giants, who have been guilty of impiety towards the true God, and towards the angels in heaven, and who have fallen from it, and who haunt the denser parts of bodies, and frequent unclean places upon earth, and who, possessing some power of distinguishing future events, because they are without bodies of earthly material, engage in an employment of this kind, and desiring to lead the human race away from the true God, secretly enter the bodies of the more rapacious and savage and wicked of animals, and stir them up to do whatever they choose, and at whatever time they choose: either turning the fancies of these animals to make flights and movements of various kinds, in order that men may be caught by the divining power that is in the irrational animals, and neglect to seek after the God who contains all things; or to search after the pure worship of God, but allow their reasoning powers to grovel on the earth, and among birds and serpents, and even foxes and wolves.
For it has been observed by those who are skilled in such matters, that the clearest prognostications are obtained from animals of this kind; because the demons cannot act so effectively in the milder sort of animals as they can in these, in consequence of the similarity between them in point of wickedness; and yet it is not wickedness, but something like wickedness, which exist in these animals.
Source: Against Celsus (New Advent)