You should improve the jumping, the graphics, and the music.
Grate Scott my horse can run over 88mph!? i hated the bgm you chose, the loop is under 45seconds and sounds like it was built with in 8bit chiptoon program. out side of that the game had a vary comfortable Atari 2600 look and feel it seems to me that a changing environment would have helped the game as well as some more obisticals to choose form, maybe a flat stretch that could spawn with a turned over wagon or a hanging sign that if you jump into ends your run. sorry i don't have more to offer. I'll spend a few more monets thinking about this and if i come up with anytthing i'll PM you.
not original at all but good game for what it's worth
graphics are nice, but the gameplay could improve
poor jumping of the horse , weird foreground and weird running of the horse .