The Spawner! These would be extremely rare, humanoid mobs, spawning in the overworld. Like Endermen, they would pick up a certain item from the world. However, The Spawner would 'pick up' mobs, any mob, whether peaceful, neutral or hostile, and walk around with them, acting like a monster spawner block. However, they would probably have varying spawn rates depending on the mob they hold. These mobs would act as a way to obtain spawn eggs in Survival Mode, dropping the mob they were holding. They may also have a chance of dropping their own spawn egg, especially if they weren't holding another mob. They would have to be a hostile mob, as finding one that creates infinite peaceful mobs could simply be used to easily farm mobs. This is why the spawn rate for the mob they hold would have to be quite low, also making it easier to defeat if it held a difficult hostile mob. Design for this creature has absolutely endless possibilities, so I think this is more of a concept than an actual mob. Depending on the design it could have higher chances of spawning in certain biomes, could have a preference towards certain mobs, and even just deciding how it 'holds' the mobs would be interesting too. It would be great to model it after the monster spawner, or at least give an origin to the monster spawner, or a way to craft it. (Hence why The Spawner - which could also use a more creative name - would be quite rare). The monster spawner could also be crafted using a key component from The Spawner in the centre surrounded by iron, or perhaps another metal or material that is dropped by The Spawer.
Thank you for reading my little spiel,