Paper/nms-patches/ItemMonsterEgg.patch

--- a/net/minecraft/server/ItemMonsterEgg.java
+++ b/net/minecraft/server/ItemMonsterEgg.java
@@ -109,6 +109,12 @@
     }
 
     public static Entity a(World world, int i, double d0, double d1, double d2) {
+        // CraftBukkit start - delegate to spawnCreature
+        return spawnCreature(world, i, d0, d1, d2, org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason.SPAWNER_EGG);
+    }
+
+    public static Entity spawnCreature(World world, int i, double d0, double d1, double d2, org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason spawnReason) {
+        // CraftBukkit end
         if (!EntityTypes.eggInfo.containsKey(Integer.valueOf(i))) {
             return null;
         } else {
@@ -123,8 +129,13 @@
                     entityinsentient.aK = entityinsentient.yaw;
                     entityinsentient.aI = entityinsentient.yaw;
                     entityinsentient.prepare(world.E(new BlockPosition(entityinsentient)), (GroupDataEntity) null);
-                    world.addEntity(entity);
-                    entityinsentient.x();
+                    // CraftBukkit start - don't return an entity when CreatureSpawnEvent is canceled
+                    if (!world.addEntity(entity, spawnReason)) {
+                        entity = null;
+                    } else {
+                        entityinsentient.x();
+                    }
+                    // CraftBukkit end
                 }
             }