Does anyone know how to catch an Muibbit on the secret route between route 23 (Shoal Waterway) and Route 50 (Crag Cliffs)? This guy is legitmentaly difficult to catch as hell, but I guess it's a secret Pokemon.

I've tried several attempts, including using a PokeNav and looking for it at Day and Night in Fern plant patches but it doesn't seem to work for some reason.

What am I doing wrong /vp/?