[Tuor of Gondolin]

There are some rather curious brief comments about ents in "Letters", which also indicate JRRT hadn't formulated a final concept of them.

In #247 there seems to be a suggestion they might be some form of minor Maia:
".....the Ents were either souls sent to inhabit trees, or else they slowly took the likeness of trees owing to their inborn love of trees."

# 338 does indicate they didn't die a "natural" old age:
".....Ents and their wives being rational creatures would find some 'earthly paradise' until the endof this world: beyond which the wisdom neither of Elves nor Ents could see. Though maybe they shared the hope of Aragorn that they were 'not bound foe ever to the circles of the world and beyond them is more then memory.
Although this passage would indicate ents were not neither maia nor elf or human related.

And in LOTR it seems the indication is that ents did die de facto by becoming virtual trees?????????????