"But against all the children of Israel, no dog shall whet its tongue" [Bo 11:17]
R' David Bodnick, one of the outstanding disciples of the Alter of Novaradok, was taking a stroll. Along the way, he passed the estate of an extraordinarily wealthy family.
There was a ferocious dog chained to a pole near the gate of the mansion to deter intruders. When the dog saw R' Bodnick, it began to bark wildly and it pulled on its chain. The chain was not strong enough and it finally snapped. The dog charged at R' Bodnick.
The master of the estate saw what was happening and realized that this was a life-threatening situation. He attempted to restrain his dog, but it was of no use, the dog overpowered its master and continued its charge towards R' Bodnick.
This dog is simply following the will of its Creator, thought R' Bodnick. I too must now do the will of my Creator - I must now arouse myself to repent.
R' Bodnick did just that. Amazingly, the fearsome dog calmed down and meekly returned to its pen.
Source: Rabbi Yisrael Bronstein