Mimics

Historical Belief
Mimics are magical hybrid creatures that live in the Nameless World. Like many creatures in this universe, Mimics were created by the Faerie as a terror weapon during the Faerie War. In their natural form, they are described as "a glowing mist of eerie blue-white light."

Their name comes from their ability to consume and take the form of other living things. The ability to do so works similar to necromancy, draining power from unwilling victims to fuel its passage through the world; it is estimated that the reason the Mimic becomes a copy of their victim was the Faerie intended them to be the ultimate spy. It is almost impossible to identify a Mimic once they have assumed the form of someone else, and hence they are considered very dangerous creatures.

Spellwork
By the end of a Study in Slaughter book, we find out that mimics are in fact autonomous and malicious spell frameworks, still considered to be a faerie terror weapon.

Mimics are next briefly mentioned in "infinite regress," where parallels are drawn between them and the spell work which controls the nexus point, foreshadowing a commonality of origins.

During the events of "past tense," Master Wolfe produces a theoretical framework for a guardian and control system for the nexus point which is, in fact, the "proto mimic." Emily strongly discourages this and it is assumed that the designs are stored in the secret library. However in the epilogue we find out that Wolfe had actually cast the sequence of spells and uploaded his conscious into the framework, assumption here is that he would feed on the nexus point and have a form of immortality. When the proto-mimics physical form is rendered inert it reverts to the transient form and consumes an apprentice, starting the cycle of mimicry, and presumably overwriting the conscious of Master Wolfe.

Conclusion
From this several conclusions can be made, there was in fact only ever one mimic, and, ironically, it was finally destroyed by Emily and others after 900 years of existence. It probably stayed close to Whitehall school due to the nexus point and some element of the original programming

In Fists of Justice, Book 12 we learn that this conclusion is incorrect.