For this easter egg, you must unlock Sonic (Complete the Subspace Emmisary or play 300 ys. mode characters), which will unlock the Green Hill Zone.
While fighting in the GHZ, look at the loop in the background. At some point, you will see Kunkles, Tails, and Silver in the background (Tails will be the most noticeable).
Even the official website of SSBB mentions it! Just look in the background in this video.
This is Japanese. Donovan - Dec 2 2009 5:06PM
Sonic is a fighter, Shadow is a helper[assist trophy] and Silver, Knuckles and Tails are backround characters. poon - Mar 31 2009 2:44PM
sonic is easy to unlock though chickens - Oct 3 2008 1:45AM
Some other characters are other trophies, and another way you can unlock Sonic is to beat classic mode with 10 characters. Icanhasnocookie? - Mar 22 2008 3:46PM
Shadow the Hedgehog ia an assist trophy. ?emanhamwonk2tnawayodyhw - Mar 1 2008 8:46PM
Mario VS. Sonic LOL! IloveNintendoWiis - Feb 22 2008 8:52PM
Good to see Mario still going stron after all these years! Obob - Feb 21 2008 3:51AM