Dennis Reynolds, Jr. was a former pet dog of Mac and Dennis'. He was purchased by Dennis when the two moved to the suburbs and was named by Mac.
Dennis, Jr. only appeared in the episode Mac & Dennis Move to the Suburbs, where he spent around 2 weeks with them. When Dennis bought him, he also made a list for Mac to take care of the dog. But since Mac was already tired due to the random "beeps" he couldn't take care of Dennis Jr. After a day of work Dennis witnesses Dennis Jr's grave being dug by Mac because his neglect had caused the dog to die of starvation. Dennis was too busy fighting with a hallucination of his neighbor, Wally. When Mac and Dennis go inside, they eat some mac and cheese which was revealed to have some of Dennis Jr's corpse inside of it by Mac.