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Unknowns are mobs that don't quite fit into any tier. They're easily the most diverse tier, as the mobs in it have almost nothing in common. They all spawn in different ways and share nearly nothing in common. They range a lot in power and unique abilities. There are a total of eleven mobs in this tier.
Unknown Mobs
Spawning
Scamper - Has a 10% chance to spawn instead of an Evolved mob when an Infected mob evolves.
Gastgeber - When a Scamper's ten minute mound age timer finishes, if it has seven kills it will become a Gastgeber.
Has a 30% chance of spawning from a linked Scamper, regardless of how many kills it has.
Spawned from Arena Tendrils as a special mob during a raid.
Specter - Has a 30% chance to evolve from an Infected Wandering Trader when it is linked.
Spawned from Arena Tendrils as a special mob during a raid.
Infested Construct - Spawned when an iron golem dies with Mycelium Infection, which will spawn its broken version on the floor. Every 10 seconds, it checks for linked Infected, Evolved, and Experiments within a ~32 block radius and tells them to come towards the broken golem. If one of the mobs gets within ~5.4 blocks, they will enter the golem and pilot it, spawning the Infested Construct.
Spawned from Arena Tendrils as a special mob during a raid.
Hallucination - Spawned from a Delusioner or the Madness effect.
Claw - Has a chance to drop from an Infected Evoker when it is damaged.
Scent - Has a 5% chance to drop from every spore mob (excluding Hallucinations and Calamity body parts.)
Calamity Body Parts - Drop from their respective Calamity (e.g.: Sieger Tail drops from the Sieger when it is damaged enough.)
Arena Tendril - Spawned when a Brain Remnant is lit on fire.
Shared Aspects & Capabilities
Unknown mobs will attack mobs with Infected forms, golems, and most hostile mobs (excluding Scents and Calamity limbs.)
Unknown mobs inflict Mycelium Infection for 30 seconds when dealing melee damage (excluding Hallucinations, Scents and Calamity limbs.)
Hallucinations inflict Mycelium infection III for twenty seconds.
Present a weakness to the cold, like all spore mobs.
Trivia
Unknown mobs are also called anomalies, due to their spawning nature.
Calamity limbs are referred to as 'FallenMultipartEntities' in the code.
Gallery
A group of Hallucinations spawned by a Delusioner.