Who or What is Tom Bombadil?

[Raynor]

Treebeard is only called the eldest among Ents:

Originally Posted by The road to Isengard, TTT

For Treebeard is Fangorn, and the eldest and chief of the Ents, and when you speak with him you will hear the speech of the oldest of all living things

In letter #153, Tom is called "the Eldest in Time"; also, in a draft letter which appeared in The Lord of the Rings Companion, by Hammond and Scull, it is stated that:

Eldest was the courtesy title of Treebeard as the oldest surviving Ent. The Ents claimed to be the oldest speaking people after the Elves until taught the art of speech by the Elves...They were therefore placed after the dwarves in the Old List... since Dwarves had the power of speech from their awaking

Even in Treabeard's song, in the Treabeard chapter, TTT, he doesn't claim he is the eldest being:

Learn now the lore of Living Creatures!
First name the four, the free peoples:
Eldest of all, the elf-children;
Dwarf the delver, dark are his houses;
Ent the earthborn, old as mountains;
Man the mortal, master of horses