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After obtaining the Landmaster's Flight Upgrade, a secret cutscene can be triggered by flying it up to a portal found on Titania.
After going through the portal a secret mission will start, with the player taking control of Peppy.
Peppy with say "barrel roll" whenever performing the maneuver, most likely as a nod to his "Do a barrel roll" line from Star Fox 64.
Contributed by CuriousUserX90
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The Arwing transforming into a Walker is a concept taken from the cancelled Super Nintendo game Star Fox 2.
Contributed by CuriousUserX90
Nintendo wanted to release for the game a special amiibo figure of an Arwing which could also transform into the walker, however it was tough to accomplish in standard amiibo size and it wouldn't have met product-safety standards.
Contributed by KnowledgeBase