Lost old favorite

Hi new here as you can see.
Ive lost a game,and cant stand it, i think it was sega mega drive (was in millitairy service at that time),
and it belonged to a pall of mine. it was the year late 1990, mid 1991 when i played it.

It was a platform + adventure game + puzzle.
The only thing i remember is that there was a level where you were a fish in a underwater-castle.
You had to collect items and return them to their owners or perform tasks for people.

You would morph in to a new creature when you completed a "complete level". Not sure, but i think you could morph in to the creatures from before, to try to collect the items.

Hope someone can me with this one.
Greetz