Non horses such as llamas are not using their appropriate Inventory object in things such as InventoryClickEvent

 

This is because it is hard set to CraftInventoryHorse on line 24 in ContainerHorse

This should be changed to 

org.bukkit.inventory.Inventory inventory = ((org.bukkit.inventory.InventoryHolder) this.f.getBukkitEntity()).getInventory();

 

So that the appropriate horse inventory is used.