The Ents were as different from one another as trees from trees: some as different as one tree is from another of the same name but quite different growth and history; and some as different as one tree-kind from another, as birch from beech, oak from fir.
Lord of the Rings: The Two Towers, p. 83
The Treants of Orn are similarly diverse.
In all likelihood, the first Treant the adventurers encounter will be the Rowan Treant, that shepherds the rowan trees near the Gnome Domes. He resembles Tolkien's Quickbeam:
Quickbeam appeared to be one of the younger ents. His hair was smooth, his lips ruddy and his hair green. He was tall but could bend and sway in the wind like a slender tree. His voice was resonant and higher and clearer than Treebeard's
Elsewhere the characters may encounter male Treants that remind them of sturdy Oaks, ancient Yews, tall Beeches or slender Ashes. Indeed, almost every tree on Orn is represented among the Treants.