I liked Serenity's Reavers. Not zombies, but also way creepy and incredibly dangerous.
Not as creative as giant thong-armed snakes, but still dangerous. And fitting, if you happen across derelicts. They'd be an awesome NPC bad-guy faction, perhaps headed by a nefarious solenium-rift-jumper who happened upon a way to control them.
And they'd be a go-to mission target. Nobody feels bad about taking down creepers who defile/kill/skin/eat in the wrong order.