Treebeard is Treebeard because his name was too long in Entish for the hobbits to use. And Mount Doom has plenty of names in Lord of the Rings, it's just that Mount Doom was its name in the Common Speech.
My mom does this same thing! She once started talking to me about a mini Jessica Simpson that she liked to put in her pocket. I just kept talking to her, it was hilarious!<<<< Wow XD these sound like great conversations