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.

If there are fewer than X cards in that player's library, put the creature on the bottom of that library.

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.