Take the frog back to its home; the Ol' Pond.
When you are mowing the pink grass with your weapon in Village of All Beginnings or Forest of All Beginnings, a frog will appear and start hoping around. Leap on it and it will attach to Tomba's back. If Tomba takes any damage, the frog will jump off and disappear. Mow down some more grass to make it reappear.
Now carefully make your way over to the Ol' Pond. When you are there, walk towards the pond and the frog will automatically jump into the water as soon as Tomba gets close to the water. The event is cleared.