A treant is not a magical creature in any way. It is a plant. Specifically, it is a tree. It is a type of tree that does not exist in the real world obviously, but in the magical world of D&D, it is a “huge plant.” It says so right there in the monstrous manual just below “TREANT”
That’s right folks, a plant. Now, “plant” is in fact a broad term used in this game for classification, and can be divided into sub-categories like flower, tree, or weed. The treant is very clearly a type of tree, with trees being described as “A woody perennial plant having a single usually elongate main stem generally with few or no branches on its lower part,” by Webster’s Collegiate Dictionary. The Treant in Monstrous Manuel fits this description nicely. Its is a tree.
As such it is a legal target for the Awaken spell, which would immediately add 2hit dice and d3 charisma, and someone would have to roll 3d6 to find out its new intelligence score, which may be more or less than the standard 12.