Shapeshifter
Shapeshifters are a loosely-defined group of beings capable of assuming other shape and forms, often imitating other beings. They can be found in all colors of mana, but predominately in blue.
Dominarian Shapeshifters
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Vesuvan Shapeshifters
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Kamigawa Shapeshifters
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Lorwyn-Shadowmoor Shapeshifters
Changelings
Changelings are relatively harmless, semi-sentient creatures native to Lorwyn. They can be difficult to find, but they are quite distinctive when they are found, given their appearance. A changeling's standard appearance is that of a squat, greenish-blue humanoid with slightly scaly skin, large eyes, a translucent body, and frequently a forked tail. Their most noteworthy attribute is their involuntary ability to assume the shapes and characteristics of the creatures around them. This occurs no matter what type of creature is around, to their pleasure or annoyance, and sometimes it can result some truly dramatic changes, ex. Changeling Titan. It is believed they reproduce in the secret cave of Velis Vel.
Mimics
After the Great Aurora, the changelings were transformed into Shadowmoor's mimics, aggressive and vicious creatures that attack in packs while taking forms that resemble warped reflections of their victims.
Pucas
Found only in Shadowmoor, pucas are horse-headed shapeshifters whose moods swing wildly from mischief to malice. They are amphibious and are particularly common near flooded swamps, mountain streams, and waterfalls.
Mirrodin Shapeshifters
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Rathi Shapeshifters
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Ravnican Shapeshifters
Lupul
Within Ravnica, a group of amoeboid insects called lupul can fuse together to impersonate other beings. Thought to have been defeated centuries before the Decamillenial, they resurfaced under Szadek's commands, until his control was disrupted by Jarad. They resurfaced years later impersonating the nephilims of yore.