Custom mob spawner (old)

1. Open the entity editor to edit the entity you want to spawn.
2. If you want, customize the entity and apply the changes.
3. In the Tools category, copy the entity to the IBE clipboard.
4. Close the entity editor and open the block editor to edit the mob spawner.
5. In the Spawn Potentials category, replace the current entity by clicking on Clipboard.
6. Select the entity you just edited.
7. Apply the changes and you're done!

Note : the first entity spawned will be the default one (a pig). If it doesn't spawn after a few seconds, in the Mob Spawner category, replace the pig by clicking on Clipboard, selecting your entity and applying the changes.

More information about mob spawners

Each mob spawner contains a list of spawn potentials. A spawn potential is a weighted entity. The higher the weight, the higher the probability of the entity to spawn.

The mob spawner also contains the data of the next entity to spawn. Every time an entity spawns, the spawner determines which entity to spawn next, depending on their weight, and caches the entity data into a separated field. This is why you can edit the next spawn data in the Mob spawner category : it will only apply for this entity, and won't apply for the ones that spawn after.

For more information about spawners, see this page.