Wagon Horse was Seven's horse in the past which was revealed in the Adventure Time episode "Walnuts & Rain." It pulled a wagon while Seven sat at in the wagon and transported Seven through the Land of Ooo.
One day, Seven was on the way to the Candy Kingdom with goods in his wagon when suddenly the horse began to act wild and crashed the wagon into a hole. Seven and the wagon fell into the deep hole and the horse was left above the Grass Lands.
Wagon Horse looks like a normal horse. In the episode "Walnuts & Rain," it was tied from the mouth with a rod to a wagon and also had a large wooden leash over its neck. It also had a blue rag over his back with yellow zig-zag stripes.