Look Who's Barking Charmed : Season 3 Episode 21

Phoebe has a premonition of a murderous banshee while adding on to the Belthazar section of the Book of Shadows, thus causing her to think Cole is responsible. But when the sisters cast a spell it backfires and turns Prue into a dog, and when Prue gets hit by a car, Phoebe's remorse causes the banshee to attack her, turning her into a banshee.