The frog is unlockable in the full game. But it's a secret.
I saw something that’s shining but isn’t very bright in the plane after the song ended. I took the artifact and then the brightness disappeared . The brightness was near the place where the ghost was living on . Either my computer screen has some problems or that’s the scret place of the frog . Excepting the screen problems, I need to know how to let the brightness appears again because I only did it once when I was arbitrarily playing the game.