If you target an opponent's creature with Dragon Turtle's Drag Below ability and that target is illegal as the ability tries to resolve, the entire ability is removed from the stack and does nothing. Dragon Turtle will remain untapped and will untap as normal during its controller's next untap step.

If, however, there are no legal targets for the Drag Below ability when Dragon Turtle enters the battlefield (or you choose not to target anything), Dragon Turtle's ability will still resolve and cause it to be tapped and it won't untap during its controllers next untap step.