When Sonic discorvers the Merpool and decide to swim on it in the full moon, he transform into a merhog, he got scared and he have to to keep it as a secret.
It was a fight that lead to Sonic's disappearance. This wasn't a normal fight, it was floating over his weakness... water. When Sonic's friends can't get him out from the water in time, each gives up over time. If he's still alive, is he wounded? How did he avoid drowning? And if he didn't go home, where is he?