Sings the Rooster!
The rooster is the herald of the dawn. So when Jesus told Peter he would deny him three times before the cock crows, He meant he would deny Him three times before the morning.
Peter’s third denial occurred around a charcoal fire at the home of Caiphas, chief priest of the Jews. Immediately, after this, the rooster crowed.
A charcoal fire is only mentioned twice in the New Testament. This was the first time. The second time occurred after Jesus' death and resurrection when He comes to the apostles on the seashore after He directs them to a miraculous catch of fish. This second time He asks Peter three times “Do you love me,” thus allowing Peter an opportunity for forgiveness.
Because of this, the rooster has come to represent God’s mercy and forgiveness.
***Thanks to Judy Collins, for sharing this information from Dr. Tim Gray's class, "St. Peter," on Formed.