Iriomote is wonderful. Been twice, can't get enough of it. Been in March and August and March is definitely great for trekking, but August has tons of critters out that are quite amazing.

There is a ryokan with an old lady who knows everything about the island and who's seen the Yamaneko a few times. The Jungle something hotel or other had Yamaneko around it. They regularly post pictures on their facebook from customers who saw one.

Me? Nope, never saw one, but I only found out about those after my second time there :)