The Jack-in-the-Box (びっくり箱, Bikkuri Bako, literally meaning “Surprise Box”) is a demented-looking, stitched-up clown creature that resides in a box located in Hogback’s room. In the manga, it has several nails embedded in its head while in the anime, it has none. It was discovered by Nami, Usopp, and Chopper when they opened the box in hopes of finding treasure. It has the number 256 printed on its collar. While the box it was in had nothing else in it, it however commented that a treasure of some sort is hidden somewhere else on the island. This comment interested Nami very much.

(Source: One Piece Wiki)