EntsThe Ents (?) - The Ents are amongst the oldest living things in Arda.  It's not exactly clear when they were created, but created they were by Eru (Ilúvatar) at the behest of Yavanna in order to protect them from other creatures, particularly the Dwarves who had been created by Aulë.

 

"But Manwë rose also, and it seemed that he stood to such a height that his voice came down to Yavanna as from the paths of the winds.  He said, '...But in the forests shall walk the Shepherds of the Trees.'" ~The Silmarillion, chap. 2 (Of Aulë and Yavanna)

 

And so long were their lives that some of them survived even into the Third Age of the World.

 

"'Only three remain of the first Ents that walked in the woods before the Darkness; only myself, Fangorn (who is Treebeard), and Finglas and Fladrif--to give them their Elvish names, you may call them Leaflock and Skinbark if you like that better.'" ~The Two Towers, chap. 4 (Treebeard)

 

By the Third Age their existence was thought to be a myth.

 

"'The Ents!' exclaimed Aragorn.  'Then there is truth in the old legends about the dwellers in the deep forests and the giant shepherds of the trees?  Are there still Ents in the world?  I thought they were only a memory of ancient days...'" ~The Two Towers, chap. 5 (The White Rider)