When Quarry Colossus enters the battlefield, put target creature into its owner's library just beneath the top X cards of that library, where X is the number of Plains you control.
If there are fewer than X cards in that player's library, put the creature on the bottom of that library.
Quarry Colossus's ability is mandatory. If you control the only creatures when that ability triggers, you must choose a creature you control (which could be Quarry Colossus itself) as the target.
The value of X is determined as the ability resolves. If you control no Plains at that time, put the creature on top of that library.