This language, known today as Entish, was strung together using Elvish words in polysyllabic phrases filled with near-endless adjectives and the (very) occasional noun. This is a language of long, slow observation, and a very tree-ish reluctance to jump to conclusions.
The Onodrim physically resembled most the family of tree they were descended from. Their strength is overwhelming. It's similar to how over time a tree's roots can destroy even the toughest fragment of sidewalk. The only difference being is that the Ents displayed this kind of power in only a few seconds.
In their early history (known as the Elder Days), the Onodrim spoke with the Elves (very quickly, as opposed to with eachother was was quite slow) quite often. Eventually the Ents broke away from the Elves.
Near the end of the First Age, men were growing in population and power throughout Middle-Earth. The Onodrim felt it necessary to retreat deep into the forests to live in secrecy.
When the War of the Ring came, near the end of the Third Age, the Ents still lived secretly in the forest Fangorn. They wandered alone, slowly dwindling.