Ropes were items that were created by placing five pieces of String in an X shape. It was used to lead tamed mobs around.
In the newest version of the Mo' Creatures mod for 1.6.2, Ropes no longer exist. They've been replaced with the Lead which is an item from Vanilla Minecraft. Edit
The Ropes could not attach to Fences like Leads can.
Ropes could be used to pick up tamed Kitties by their feet, though the Kitty would get annoyed if you carried them this way and they would attack you once they were released.