Click the followin' links and they'll direct ye' to some den meshes that may be of use:
http://www.onelittlecorner.proboards.com/thread/6/map-objects-new-bigcave (http://www.onelittlecorner.proboards.com/thread/6/map-objects-new-bigcave)
http://some-art.deviantart.com/art/Old-Meshes-Package-Downloads-292854612 (http://some-art.deviantart.com/art/Old-Meshes-Package-Downloads-292854612)
http://chunkymonkey94.deviantart.com/art/IceCave-mesh-298243113 (http://chunkymonkey94.deviantart.com/art/IceCave-mesh-298243113)
http://feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=8174.msg196067#msg196067 (http://feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=8174.msg196067#msg196067)
Also, in case you didn't know, FeralHeart comes with a den mesh already. The mesh is called den1.mesh, and you can texture it with anything.
Also, this is a den/cave that might fit your needs. It is also by ChunkyMonkey.
http://chunkymonkey94.deviantart.com/art/DenCave-mesh-for-FeralHeart-275832082 (http://chunkymonkey94.deviantart.com/art/DenCave-mesh-for-FeralHeart-275832082)
Toshema has a tree den you might find useful.
http://chunkymonkey94.deviantart.com/art/FH-Tree-Den-322023827 (http://chunkymonkey94.deviantart.com/art/FH-Tree-Den-322023827)