An ectoplasmic egg is a magical item which Gargamel had two in his possession in the cartoon show episode "It's A Puppy's Life". It is a crystal egg that enables its possessor to switch bodies with another possessor of an identical egg as they both look into it, allowing them to have complete control over the host body that they now possess. However, if one of the two eggs break, the persons in the switched bodies will revert back to their original bodies.
In the episode, Gargamel placed one of the eggs in the forest for the Smurfs to find so that he could switch bodies with Papa Smurf and thus lead the other Smurfs into being captured. However, as Gargamel had finished reciting the incantation for the transference, Puppy had knocked the other egg out of Papa Smurf's hand as he was looking into it and Gargamel ended up switching bodies with Puppy as the dog now looked into the egg. Although disappointed that he didn't enter into Papa Smurf's body, Gargamel as Puppy decided to play along as being a good little doggy until he started capturing Smurfs and placing them into a pit that they cannot get out of. Meanwhile, Scruple was having fun with Puppy in Gargamel's body as he ran the wizard now possessed by a dog's mind ragged through the forest. As Gargamel in Puppy's body attempted to go after Papa Smurf after all the other Smurfs were captured, Papa Smurf realized from Gargamel's own words that the egg was responsible for Puppy now behaving like Gargamel and tossed the egg outside his laboratory where it was broken. With the egg broken, Puppy and Gargamel then switched back to their original bodies.