The Labrador Retreiver or also known as Plankton's Lab, or just The Labrador is a minor character in the show.
It was only featured in the episode "Plankton!", when Plankton took over SpongeBob's mind, and introduced the labrador. The Labrador was only shown on a screen, when Plankton said "And...this is my lab!", when it was shown, it barked for a few seconds or so, before Plankton took SpongeBob to the room with Karen, so he could discover the Krabby Patty secret formula.