LivingEntity の setCustomName メソッドは、Minecraft 1.5 以上で使用できる

gistfile1.java

/*
 * @author     ucchy
 * @license    GPLv3
 * @copyright  Copyright ucchy 2013
 */
package jp.ucchy.cp;

import org.bukkit.Material;
import org.bukkit.entity.LivingEntity;
import org.bukkit.entity.Zombie;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.entity.CreatureSpawnEvent;
import org.bukkit.inventory.ItemStack;
import org.bukkit.inventory.meta.SkullMeta;
import org.bukkit.plugin.java.JavaPlugin;

/**
 * @author ucchy
 * さーかすぱらだいす
 */
public class CircusParadise extends JavaPlugin implements Listener {

    private ItemStack circus;

    /**
     * @see org.bukkit.plugin.java.JavaPlugin#onEnable()
     */
    @Override
    public void onEnable() {
        circus = new ItemStack(Material.SKULL_ITEM, 1, (short) 3);
        SkullMeta meta = (SkullMeta)circus.getItemMeta();
        meta.setOwner("circus55");
        circus.setItemMeta(meta);
        getServer().getPluginManager().registerEvents(this, this);
    }

    @EventHandler
    public void onMobSpawn(CreatureSpawnEvent event) {

        if (event.getEntity() instanceof LivingEntity) {
            switch(event.getEntityType()) {
            case ZOMBIE:
                Zombie zom = (Zombie) event.getEntity();
                zom.setVillager(false);
            case SKELETON:
            case PIG_ZOMBIE:
                LivingEntity le = (LivingEntity) event.getEntity();
                le.getEquipment().setHelmet(circus.clone());
                le.getEquipment().setHelmetDropChance(0F);
                //le.setCustomName("circus55"); ← ここはMC1.5以上じゃないと使えない
                break;
            default:
            }
        }
    }
}