paper-api/src/main/java/org/bukkit/event/entity/CreatureSpawnEvent.java

@@ -211,6 +211,10 @@ public class CreatureSpawnEvent extends EntitySpawnEvent {
          * When an entity spawns from an ominous item spawner
          */
         OMINOUS_ITEM_SPAWNER,
+        /**
+         * When an entity spawns from a bucket
+         */
+        BUCKET,
         // Paper end - Fixes and additions to the SpawnReason API
         /**
          * When a creature is spawned by a potion effect, for example:

paper-server/patches/sources/net/minecraft/world/item/MobBucketItem.java.patch

@@ -0,0 +1,11 @@
+--- a/net/minecraft/world/item/MobBucketItem.java
++++ b/net/minecraft/world/item/MobBucketItem.java
+@@ -58,7 +_,7 @@
+         }
+ 
+         if (mob != null) {
+-            serverLevel.addFreshEntityWithPassengers(mob);
++            serverLevel.addFreshEntityWithPassengers(mob, org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason.BUCKET); // Paper - Add SpawnReason
+             mob.playAmbientSound();
+         }
+     }