The Bonecoaster is a rollercoaster-like creature made of bones that is rideable. It helps Mario, Luigi, Yellow Toad, and/or Blue Toad get through 8-7 in New Super Mario Bros. Wii. It does not harm any of the characters, but makes dangerous drops into the lava below. The Bonecoaster can withstand the weight of all four characters, but if they perform a ground pound at the same time, the Bonecoaster will break, making them drop into the lava.
The Bonecoaster's name is a pun on bone, referring to what it's made of, and rollercoaster, referring to what it acts like.