From the Super Mario Wiki
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At first, a Cool Kong is twice as large as a Mini Mario. However, if a Mini Mario grabs their ties, the Mini Mario will hold on to it while the Cool Kong moves around. The pressure lowers the Cool Kong by half, and the Mini Mario can hold on as long as it pleases – this is critical to get over long obstacles, such as water and spikes.
A Cool Kong moves back and forth on the surface it's on, which is often blocked off of normal access by a Mini – except for the tie, which often must be jumped for. When a Cool Kong is lowered, it sometimes travels under another low platform, allowing it to travel farther than it could have without a Mini. Often this is the key to get that particular Mini to the door (or even Minis – sometimes the Cool Kong is able to reach a spot where another Mini can ride on top of it, like any other movable platform).
They are first seen in Magnet Mania.