Small larvae kept by Dr Phlox which, when injected with another being's DNA, grow into a short-lived duplicate known as a mimetic simbiot, as in Similitude.
The Lyssarian desert larva is one of the stranger animals in Phlox's sickbay collection, and is central to the Enterprise episode Similitude. If the larva is injected with genetic material from another species it grows into a copy of the donor, passing from infancy to old age in about fifteen days.
In 2153 Tucker is left in a coma after an engine accident while Enterprise is stranded in a dangerous field in the Delphic Expanse. Phlox proposes to grow a simbiot and take neural tissue from it to repair the damage, and Archer reluctantly agrees.
The copy, named Sim, turns out to have Tucker's memories as well as his body, and becomes a valued member of the crew within days. The transplant will kill him, and a possible treatment might let him live a full life. Sim finally chooses to go through with the operation.