Keeper (Babylon 5)
Keepers are artificial life forms found in both Babylon 5 and Mass Effect.
Babylon 5
In Babylon 5, a keeper is an organic construct used by the Drakh to control the behavior of an individual enemy. It is an octopus-like creature consisting of a small body, a few tentacles, and a single eye. It presumably derives sustenance by attaching itself to its hosts circulatory system, but the details are not known.
A keeper can control its host's behavior, but only when its own interests are at stake. consequently, they can be tricked by convincing them that the host's actions are irrelevant to the plans of the Drakh.
Like the Shadows, a keeper is invisible in normal light. They become visible, however, if injured or rendered unconscious. Londo Mollari discovered that he could put his to sleep by drinking enough alcohol.
Even if seriously damaged, a keeper will regrow on its host in a matter of minutes.
Mass Effect
In Mass Effect, the Keepers are a race of intelligent (but not sapient) arthropods that exist to maintain and service the Citadel.