Mr. Muggles is the dog that belongs to the Bennet family, specifically Sandra Bennet. He is a breeding Pomeranian who has been entered in dog shows.
Sandra speaks with Claire about her day, telling her about an offer one of her friends made to have their dog breed with Mr. Muggles. Later, when Claire accidentally severs her hand, Mr. Muggles drinks up her blood, much to her disgust. Mr. Muggles is soon entered in a competition, which he and Sandra fail to win. One day, when Sylar breaks into the house, Mr. Muggles is used as an excuse by the deranged serial killer. He tells Sandra that Mr. Muggles was running around in the street and that the back door was left open. When Sandra begins to exhibit some memory loss, Claire learns that it is far worse than anyone feared as Sandra does not even recognize Mr. Muggles. The family leaves the dog at home while they go to the hospital and Mr. Muggles becomes aware that people are breaking in. He goes to the door to see who it is and Ted Sprague and Matt Parkman enter the house. The family (Mr. Muggles included) is held prisoner inside the house. When Sandra is freed, she rescues Mr. Muggles and the entire family witnesses Claire regenerate. Later, Candice Wilmer uses Mr. Muggles as a discussion topic in order to convince Noah that she is Sandra.
Mr. Muggles goes with his family to Costa Verde where, according to Sandra, he has trouble fitting in. Sandra claims that Mr. Muggles is having trouble readjusting to normal life after spending time in a luxury hotel. Claire soon decides to test the limits of her ability and puts on a dog program to keep Mr. Muggles happy. Mr. Muggles spots West spying on them and rushes outside with Claire. While Claire cannot see West, Mr. Muggles continuously barks and leaps up at the sky. Mr. Muggles continues to live a Californian lifestyle until Noah Bennet is killed. While Sandra sits and mourns, Mr. Muggles rushes up to comfort her. Soon after, Mr. Muggles detects Noah at the door and rushes to greet him.
Mr. Muggles sits with a traumatized Claire while she waits for the family to return home. Two months later, Mr. Muggles sits with the family at the breakfast table while they discuss Claire's college hunting. He later accompanies Sandra to Claire's room, where he finds a half eaten sandwich under the bed and hears Alex Woolsly in the closet. He barks at the doors, causing Alex to fall and expose himself. Later, Sandra goes to Washington to look for Claire. Lyle calls Sandra and asks where Mr. Muggles medicine is. Sandra tells him where it is and Lyle hangs up. During the summer, Mr. Muggles is at home with Claire and Sandra and listens as Claire tells Sandra about a group of people that are in town. Mr. Muggles rushes outside and meets with Markus Gaines, who understands what he is saying. He controls Mr. Muggles and several other dogs, getting them to attack Claire, Sandra and the newly arrived Mags. While the other dogs attack the other girls, Mr. Muggles is the only one attacking Sandra, much to her shock. Markus is soon beaten and Mr. Muggles seeks forgiveness by cuddling up to Sandra.
A photo of Mr. Muggles is displayed on a table in Noah's apartment.
In her speed dating, Claire says that her mother breeds a show dog. Claire's "sign up" profile picture, as seen on Gretchen's computer, is a shot of Claire holding Mr. Muggles.
Sandra brings Mr.Muggles with her to the Thanksgiving diner at Noah's apartment. Claire is happy to see Mr. Muggles. Later, after Doug passed out, Mr.Muggles licks Sandra as Miss Lovegood licks him.
The Fifth Stage
When Claire is telling a fairy tale version of her life story to the carnival children, she depicts her family as frogs and Mr. Muggles as a tadpole.
In series two Sandra comes back from a walk and says: "You wouldn't believe it but that dog can power walk." and nobody believed it