@Izzie,
Hi Izzy,
Assuming that Francis was giving us a hint when he said...
"Well, this frog lives only on a particular island.
(You wouldn't believe it if you haven't seen it..)"
I think we're looking for an island which is associated with the idea, "You wouldn't believe it if you haven't seen it."
Do you know of any literary or movie references to an island that fits that bill?