The Shaggy Dog (1959)

This Disney comedy is about a boy who turns into a large sheep dog at the most inopportune moments. That is assuming there would be opportune moments. Young Wilby Daniels (Tommy Kirk) finds a magic ring that used to belong to the Borgia family. When he pronounces an inscription on the ring, he is suddenly able to transform himself into a shaggy dog -- though he has no control over when this is going to happen.

To his dismay, the girl he likes gets involved without knowing who the dog really is. At the same time, the only way Wilby can break the spell is to perform some virtuous deed. Fortunately for him, a few Soviet spies are just hanging around, waiting to be uncovered by a canine.