Golden Flutter Wings
From the Super Mario Wiki
Golden Flutter Wings are power-ups that first appeared in Yoshi's New Island. Its availability is like that of the Invincibility Leaf from Super Mario 3D Land, meaning that it only appears after Yoshi loses three lives in a level and loses another in the same level with Flutter Wings. Mr. Pipe gives Yoshi this power-up and it turns him into Winged Yoshi, but with infinite hovering (rather than flight) and invincibility. If all the levels are completed without the Flutter Wings or Golden Flutter Wings, then there will be a battle with Bowser before Baby Luigi and the Stork are saved.
Similar to the Invincibility Egg from Yoshi's Woolly World, Yoshi is made immune to damage by making contact on enemies, but unlike most cases of invincibility, this version of Yoshi does not defeat enemies in one touch, and he is still knocked back.