How Brackenleaf came to be is unknown, but what is known is that he is one of the great trees of Valenwood. The Briars have protected him from threats, including the Imperial Siege of Grahtwood in the First Era.
"Good, good. I think you will find this forest is full of new things to discover. Never ate a frog and found yourself talking to a tree before, did you? Of course, not all things you discover here are safe."
Brackenleaf's voice was changed. Most likely because the original voice actor did not seem to "suit" him, as one would expect a deep voice for such a large being. The original voice can be heard in this Youtube clip.