A single-celled organism that absorbs other living beings and copies their form, which killed a Starfleet officer and took his place.
A coalescent organism absorbs another living thing, slowly merges with it and then takes its exact form, so a copy can pass as the original. It can repeat the process with new victims.
In Aquiel, Lieutenant Aquiel Uhnari was suspected of killing a fellow officer at a Federation relay station. The truth was that a coalescent organism had absorbed Keith Rocha and posed as him. It later took the form of Aquiel's dog, then tried to absorb Aquiel herself before Geordi La Forge killed it with a phaser.