Bukkit

Since this idea\concept is clearly ripped off from another plugin, here's where I got it from:
http://dev.bukkit.org/bukkit-plugins/mounted-undead/

Basically, it would be interesting to not only see skeletons on skeleton-horses (zombies on zombie-horses are rather harmless, unfortunately), but also all kinds of mobs on horses and other things that would be interesting when mounted, like a skeleton riding an armoured zombie. The possibilities are endless, really.