The Shaggy Dog (1959)
Wilby Jr gets turned into a sheepdog in the original Disney film, after he uncovers an old curse by accident, which is ironic because the character just so happens to be the son of mailman Wilson Daniels, who despises dogs. Things then pick up… Not… when Wilby learns of a plot to steal government secrets.
Source
//www.youtube.com/watch?v=TiJOBf9WKBA