Chestnut: Hero of Central Park

Chestnut Hero of Central Park is a 2004 American family film, starring Makenzie Vega, Abigail Breslin, Christine Tucci, Justin Louis, Tony Alcantar, Irene Olga Lpez, Ethan Phillips, Irene Karas, Barry Bostwick and directed by Robert Vince, about a Great Dane who is adopted by two orphan girls in this heartwarming family film. The girls new family dont know about the loveable mutt however, and the film revolves around the inventive ways he is kept hidden from his new owners.

Source: Wikipedia