Birthday King Sonic's getup of the cape and crown combo began at Sonic Carnival 2014, where a Sonic statue was seen wearing the kingly birthday attire. The costume has also notably appeared in various artworks posted by the official Sonic the Hedgehog Twitter account, and even a Sonic mascot costume
Kats602Yellow crown, blue, red and green gems on it. Red cape with a white trim.
Full body reference: Birthday King Sonic — default