To unlock the Frog Fishing Lure for the Fishing Hole, you must beat all eight levels of Roal Goal, a game found in the owner's establishment. To play it, simply look at it in first person using the C button.
The game requires 5 rupees a play and you win 10 rupees for each round, and the frog lure for the eighth. (If you run short before you manage to get the lure, mowing the tall grass just outside (still in the fishing hole area) offers around 60 rupees.)
Lure description: The must-have lure for bass.