That Sonic amiibo you own isn’t useless just yet, as the upcoming Yoshi’s Woolly World for Wii U will be compatible with it. Using it will let you play as Sonic the Hedgehog Yoshi, who has Sonic’s colours and shoes. He even has green eyes!
Pretty much every amiibo, aside from Pokémon amiibo for some reason, will be compatible with the game. Each of which unlocks a new Yoshi skin based on the character. Even the original Yoshi amiibo for Super Smash Bros.  will let you unlock a skin that looks like a more normal, non-yarn Yoshi.