My friend flicka (1943)

Ken McLaughlin is a precocious 10-year-old who lives with his family on a remote Wyoming ranch. When Ken returns home from school with failing grades, his father, Rob, blames the boy's lack of personal responsibility. At the suggestion of his wife, Nell, Rob allows Ken to choose a single colt from the herd to raise as his own. Much to his father's dismay, Ken chooses a fiery mustang filly -- but the two soon become fast friends.