As a kid go to Lon Lon Ranch. Talk to Malon and she'll teach you a song. Play it to Epona and she'll grow fond of you. Then become an adult. Go to Lon Lon Ranch and pay Mr. Indigo to ride a horse. Then when you're in there, play Epona's song. Epona will run up to you and you ride her for a minute. Then pay him again, get on epona, ride to Mr. Indigo and speak to him. He'll ask if you want to race, say yes. Beat him twice. Then he'll lock you in the ranch. All you have to do is jump over the gate. To get the cow go back to the ranch and talk to Malon. She should ask if you want to do the obstacle course, say yes, and if you break the record you get a cow.