Minecraft wiki entry for Spawner, associated text: Any spawner placed in Java Edition defaults as a pig spawner.
A player-created spawner containing zombies can never spawn a Zombie Villager.[more information needed]
In Java Edition, spawners containing zombies have a small chance to spawn a chicken jockey.
Spawners containing zombies or skeletons have a chance for the mob spinning inside to have armor.
Spawners containing spiders have a small chance to spawn a spider jockey.
Spawners containing Pillagers have a small chance for the mob spinning inside to be a Raid captain in Java Edition.
Spawners in mushroom fields, mushroom field shore, or the Void biome can function normally, despite those biomes having no hostile mobs spawn naturally.
Spawners are the only block that can be legitimately obtained as an item, but not obtainable from either Creative mode or pick block.‌[Java Edition only]
It is possible for a pig spawner to generate naturally. If a newer world is loaded in an older version of minecraft, the spawner can lose its metadata, causing it to enter its default state, which is pig. Alternatively, if a buried treasure chest generates next to a spawner, it duplicates the spawner without duplicating the NBT tag, resulting in a natural pig spawner. The chest also gets invalid block (spawner) data, making it unopenable.
If a dungeon spawner overwrites a dungeon chest during world generation, the spawner does not spawn any mobs. It appears as a pig spawner, but placing a hopper underneath it reveals the contents of the chest that should have been there.