A Na'ka'leen Feeder is an alien creature from Centauri space. It feeds on neurotransmitter chemicals which causes mindwipe or death. The creature was at least quasi-sentient, and capable of communicating on a basic level through a translator. It has a tough outer skin capable of withstanding a few PPG shots (but not many).
The species was long under quarantine by the Centauri Republic, after an entire colony was wiped out on the creatures' home planet. However, by 2258, the quarantine was briefly lifted (apparently for budget reasons), allowing one to be brought to Babylon 5 by an underworld gang led by Deuce. They kept the creature inside a fake version of a Vorlon encounter suit to further terrify people into believing they had the Vorlons under their influence.
One of the feeder's victims was Miriam Runningdeer. It craved an older, more knowledgeable mind, so Deuce had his thugs kidnap the station's ombuds, Wellington. The feeder was prevented from killing him by Aldous Gajic, whose calm in the face of the danger it represented seemed to affect the feeder's behavior, prompting it to emerge. It was then killed by station security led by Michael Garibaldi. 
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