Rulings

Archelos's abilities don't apply to itself as it enters the battlefield, nor do they apply to any permanents entering the battlefield at the same time.
If another replacement effect says that a permanent enters the battlefield tapped while Archelos is untapped, the entering permanent's controller chooses whether the permanent enters tapped or untapped. If a permanent is simply put onto the battlefield tapped without a replacement effect being applied, it always enters untapped if Archelos is untapped.
If more than one Archelos is on the battlefield, the controller of an entering permanent chooses how to order the replacement effects.