Multi-mounts are mounts that can accommodate more people than just the person who summoned it. To use a multi-mount summoned by another person, press spacebar next to it. You will begin riding the mount with the person who summoned it if there is enough room. Spacebar may be pressed again to prematurely disembark before the owner of the multi-mount disembarks.
There are currently 11 multi-mounts in MapleStory 2: