From the ArcaMax Publishing, Cats & Dogs Newsletter:
http://www.arcamax.com/news/catsanddogs/s-185292-173673
Ol' Yeller, an adventurous and friendly dog, becomes friends with a
young boy -- a story that takes place on a country ranch in the 1800s.
Spoiler Warning: The yellow mutt proves himself to be worthy
of helping out on the ranch in the 1800s. But the 1957 film that
included actor Chuck Connors does not end happily. In fact, Old Yeller
is considered one of Disney's saddest films. The part of Ol' Yeller
was played by a 170 pound Labrador/Mastiff mix named Spike, according
to user comments at IMDB.com.