Ava Jayne Marie Corless was in bed at a house in Blackburn, at around 11pm on Monday when she was savaged by what police described as a large American pitbull-type dog. It has been destroyed.
Police and paramedics tried to resuscitate the girl but she was pronounced dead in hospital a short time later.
The girl's mother has been named locally as Chloe King and the man in custody, her current partner, was named as Lee Wright, who is not the father of the child.
Police have not confirmed the names of those under arrest.
They were downstairs when the attack took place, police said.
Locals claim to have made previous complaints about the dog's behaviour to police.
Chief Superintendent Chris Bithell of Lancashire Police said: "This is an absolutely horrific incident in which a baby girl has lost her life and I would like to take this opportunity to express my own personal sympathy to the wider family of Ava."
He said police were first alerted by the ambulance service who were called to reports of the girl being mauled by a pet dog.
Both suspects were initially arrested on suspicion of child neglect, before being re-arrested on the charge of manslaughter and were being questioned by detectives yesterday.