It's Sonic the...costume?
Yes! In Super Mario Maker game, you can use a Sonic Amiibo figure to unlock this "costume" of Sonic for the Mario sprite to wear. However, it's not really a costume, as much as it is apparently a Sonic 1 sprite they lay over it & give realistic movements to. Finally, everyone who wanted to see Sonic in the Mario world, this is a great chance to do it. It's a fun unlockable, & further encouragement for Amiibo owners.