A playable demo of Super Monkey Ball for Wii at a German game convention in August of last year. Writers from IGN were able to play the game which was behind closed doors. Here is a quote
"Interestingly enough, there were two new selectable characters in the demo besides the monkeys, but they haven't been announced just yet so we cannot reveal them here at this point. However, you can probably guess them for yourselves...how many SEGA mascots can you think of that would fit into this game well?"
Lets see.... its not a monkey, its a Sega mascot and the game is about things that roll
I doubt the characters were dropped from the game so could Sonic be a secret character I don't know about?