I don't know about zombies riding horses, but it would be awesome if YOU could ride a horse.
And feed it mutagen.
Why stop there? It would be awesome to be able to ride an angry moose. The attach bionics on it.
Then people can start worshiping the almighty robomoose.
Strawberry Surprise: it's a meal, drink, morale booster, painkiller, ethanol burner fuel, and coping mechanism in one gulp!