In the script for the play, Barrie specifies that Nana is a Newfoundland. (Luath, the Barries' dog at that time, was a Newfoundland.) This is confirmed in the text of the novel Peter and Wendy. However, when Barrie first conceived Peter Pan, his dog was Porthos, a Saint Bernard. read more
Nana has been portrayed in various productions of "Peter Pan" as being either a Saint Bernard or other large dog breed. In theater and live-action movies, the part of Nana has traditionally been played by an actor wearing a dog costume. read more
From lanfranco's link: The dog Nana in James M. Barrie's Peter Pan was a Newfoundland. (Newfie owners resent the depiction of her as a St. Bernard in the Disney animated film version; the 2004 film Finding Neverland used a Great Pyrenees). read more