From the Super Mario Wiki
Skeleton is an event in the Wii and DS versions of Mario & Sonic at the Olympic Winter Games. It is in the Bobsleigh category and it is similar to Bobsleigh but it can be only played individually, not with a team. Big Bullet Bill is battled in this event in Individual Festival mode only. The controls are the exact same in the Wii version, as Bobsleigh, in the DS version, the stylus is used. It is the exact same controls as Alpine Skiing GS on DS with one more addition: slide up to boost.
The event returns in the Wii U version of Mario & Sonic at the Sochi 2014 Olympic Winter Games.