While I'm less concerned about horse theft, I'm looking for a way to lock the armor and saddle on horses. Maybe let the horse be "claimed" by a player by naming it, then only that player will be able to add or remove saddles and armor from it. Something like that? Its less important that the horse be protected than the armor and saddle on the horse. Is there a plugin that does that?
Yes there is, just search Horse in the plugin directory and you will find a few good ones. Personally I use http://dev.bukkit.org/bukkit-plugins/myhorse/ for my server. Hope i helped!