Plankton's lab is a minor live-action character that appears in the episode "Plankton!" He can been seen on a massive TV in the Chum Bucket.
In the episode "Plankton!," it is revealed that Plankton has a pet Labrador Retriever. As he is controlling SpongeBob via mind control, they walk into a room and Plankton says "This is my lab!" rather loudly. In the room, there is a large theater-like screen showing video of a Labrador above water barking in a seemingly suburban area. This character also appears in the console version of video game Nicktoons Unite!
The end credits of the episode list the dog as "Alex."
This is a pun as Plankton was taking SpongeBob to his laboratory, he walked in this room and said "This is my lab!" and then he walks over and says "This is my laboratory!"
The dog is specially a Labrador Retriever which is a type of gun dog.