It seemed like just a regular rug at first, but turned out to be some kind of flat zombie creature—something the crew discovered when they ran into it during their fight with the painting zombies alongside Nami, Usopp, and Chopper.
When Luffy and the rest of the Straw Hats made it to the mansion and clashed with more painting zombies in that same dining hall, Rug Bear didn't last long. Luffy took it down with Gomu Gomu no Bazooka, and afterward made a pretty funny comment about how if they weren't on opposite sides, he'd have asked it to join the crew.
The creature popped up again later as one of the surprise zombies, freaking out along with the others when Luffy sent Oars flying.
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