Creature â Hippogriff
Creatures entering the battlefield don't cause abilities to trigger.
John Severin Brassell
Triggered abilities use the word â€œwhen,â€ â€œwhenever,â€ or â€œat.â€ Theyâ€™re often written as â€œ[Trigger condition], [effect].â€
The trigger event doesnâ€™t have to specify â€œcreaturesâ€ entering the battlefield. For example, Amulet of Vigor says â€œWhenever a permanent enters the battlefield tapped and under your control, untap it.â€ If a creature enters the battlefield tapped and under your control, Amulet of Vigorâ€™s ability would not trigger. If a land (that isnâ€™t also a creature) enters the battlefield tapped and under your control, Amulet of Vigorâ€™s ability would trigger.
Look at the permanent as it exists on the battlefield, taking into account continuous effects, to determine whether any triggered abilities will trigger. For example, if you control March of the Machines, which says, in part, â€œEach noncreature artifact is an artifact creature,â€ each artifact will be a creature at the time it enters the battlefield and will not cause triggered abilities to trigger.
If Hushwing Gryff and another creature enter the battlefield at the same time, neither creature entering the battlefield will cause triggered abilities to trigger.
Hushwing Gryffâ€™s ability stops a creatureâ€™s own enters-the-battlefield triggered abilities as well as other triggered abilities that would trigger when a creature enters the battlefield.
Abilities that create replacement effects, such as a permanent entering the battlefield tapped or with counters on it, are unaffected.
Abilities that apply â€œas [this creature] enters the battlefield,â€ such as choosing a creature to copy with Mercurial Pretender, are also unaffected.