This is what happens if a Redead, Gibdo, or Dead Hand arm grabs Link off of Epona's back. You wind up controlling both at the same time, and it's random who the camera focuses on. Here, the camera is following Epona, as you can tell. Oddly, Epona can only take damage when Link is riding her, but she can still damage enemies alone like this. Here's something interesting to try: position Link so that there is an enemy between him and Epona. Then, play Epona's Song. When Epona runs over to Link, she'll ram into and damage the enemy on the way. This is a really impractical way to attack, though, so it's mainly just for fun.