Physiology
Mimics are a highly adaptable, carnivorous variant of the "slime" class of monsters. While most slimes are simple scavengers or herbivores, Mimics have evolved a more efficient means of luring their prey by transforming into objects desired by their prey. Using a complex network of chromatophores and an amorphous, muscular layer with microscopic "locking" fibers that allow it to retain a given form without expending energy, a Mimic is capable of transforming into nearly any stationary shape, though they cannot accurately copy complex movement beyond wiggling or waving.
Unlike most slimes, which have at best a small ganglion for coordinating movement, Mimics possess a large, complex brain at the center of their mass. This allows them to learn from observation and experience, which is essential for their unique hunting strategy.
Hunting
Mimics are quite intelligent, and are believed to possess a rudimentry theory of mind, which is applied heavily in their hunting strategy. When a Mimic sees an animal it wants to eat, it will stalk the intended prey, transforming into objects in its local environment to stay hidden. It will watch the prey and try to ascertain which kinds of objects they are attracted to, and will learn to copy these forms to attract their prey.
Common Forms
The forms that a wild Mimic will usually adopt include rocks, dead animals, small shrubs, fallen fruits, and patches of land covered in grass, depending on what it is hunting. Usually a Mimic will transform a part of its body into the "lure" while camouflaging the rest as part of the landscape, allowing them to appear smaller than they actually are. Some Mimics have learned to turn a part of their body into a spider-like "lure" at the end of a stalk, which they will wave around to attract birds and insectivores, like the Spider-Tailed Viper.
As the forms they take are learned, rather than instinctive, the forms adopted by Mimics living in one environment will differ heavily from those living in different environments, even if the Mimics themselves are closely related.
Parenting
Learning to mimic objects properly takes time and effort, during which the young Mimic is vulnerable. Because of this, Mimics possess a strong parental instinct. A young Mimic will instinctively try to copy objects in its local environment; parental Mimics will facilitate this learning process by surrounding their children with objects that they have learned are useful to copy.
Mirror Learning
Mimics can recognize themselves in a mirror from an early age, and a parent Mimic will transform a part of its body into a mirrored surface so that the child can see what it is doing as it learns to use its transformative abilities. Mimics have also been known to treasure reflective objects, both for teaching their offspring and to refine their own mimicry skills. Access to a high-quality mirror greatly accelerates the Mimic's ability to master new forms - without one, the Mimic is forced to rely on extending an eyestalk away from their body in order to observe themselves, which can introduce inaccuracies in the process.
Domestication
While it is theoretically possible that Mimics living among humans may learn to lure humans using forms like gold or treasure chests, this is a poor hunting strategy - Mimics that learn to hunt humans are often targeted for destruction, like most man-eating animals. In reality, most "treasure chest Mimics" are actually domesticated - they were intentionally trained to adopt that form to catch would-be thieves.
Due to their instinct to adopt nearby forms when young, it is quite easy to train a Mimic from an early age - the trainer simply places the objects to be copied in the young Mimic's vicinity, and rewards them with food when they adopt the form. The Mimic learns to associate the "treasure chest form" with "food" in the presence of humans and will subsequently adopt that form when a human approaches.
