Ixidron

Creatures turned face down by Ixidron are 2/2 creatures with no text, no name, no subtypes, no expansion symbol, and no mana cost. These values are copiable if an object becomes a copy of one of those creatures, and their normal values are not copiable.

The controller of a face-down creature can look at it at any time, even if it doesn't have morph. Other players can't, but the rules for face-down permanents state that "you must ensure at all times that your face-down spells and permanents can be easily differentiated from each other." As a result, all players must be able to figure out what each of the creatures Ixidron turned face down is.

If Ixidron and another creature are entering at the same time, the other creature enters face up.

Turning a face-down creature face-down typically has no effect; the creature's status is unchanged.

You turn the creatures face-down *as* Ixidron enters. There is never a moment when Ixidron is on the battlefield and the creatures are face-up. If a creature on the battlefield has a "whenever another creature enters" ability, it won't trigger because that creature will be face down before Ixidron enters.