A Lyssarrian desert larvae
The Lyssarian desert larvae is a limbless ovoid larva native to the Lyssarrian homeworld.
Its epidermal layer secretes a viral suppressant that is useful for treating cuts and bruises. It also has a more obscure property: if it is injected with DNA from another organism, the larva rapidly transforms itself into a clone of that organism that grows, matures, and dies in 15 days. These clones are called "mimetic simbiots" and are practically identical to the original organism, including, in the case of Humans, having its memories.
Doctor Phlox kept a Lyssarian desert larvae aboard the Enterprise, which he used in 2153 to create a clone Commander Charles Tucker III. He then extracted neural tissue from the clone, named "Sim", to save the life of the original Tucker. (ENT: "Similitude")