Meet the Annakazi
Once the scourge of the Galactic Arm, the Annakazi are now just small packs of marauders that roam the Arm and attack anything that moves. Any survivors are taken in for food or for slave labor. Any equipment or resources are quickly "consumed" by the recyclers and they move on.
One Annakazi by itself is a kind of docile creature, semi-articulate and can play nice, for a time. As more Annakazi get together, the aggression comes out. This Locust-like mentality is what ultimately led to the Empire's downfall - so many Millennia ago...
Occasionally, these bands meet, and when they don't kill each other on sight, well that's when it REALLY gets bad. Swarms of Annakazi can lay waste to entire planetary systems. The larger the swarm the more aggressive they get.