Entwives

[The Might]

As this quote

A few [of the Ents at the Entmoot] seemed more or less related to Treebeard, and reminded them [Merry and Pippin] of beech-trees or oaks. But there were other kinds. Some recalled the chestnut: brown-skinned Ents with large splayfingered hands, and short thick legs. Some recalled the ash: tall straight grey Ents with many-fingered hands and long legs; some the fir (the tallest Ents), and others the birch, the rowan, and the linden.

shows they resembled the treeds, they reminded the Hobbits of the trees. Clearly there would be a difference between the Ents and the trees, but possibly this difference would diminish if the Ents grew treeish.

So it seems likely to me that it was due to their special assignment as shepherds and not as normal trees that they looked different, when they abandoned it they blended in with the others.
I know this sounds a bit strange, but as long as you have skin-changers capable of transforming into bears I don't see why Ents wouldn't be capable oftaking the shape of the trees they cared for.
Sounds quite like something Yavanna would think of.

[Gwathagor]

Yes, I suppose so. Though, it does say that the Ents recalled certain trees, which implies a fairly weak level of resemblance.

[Aaron]

I always found that to be tragic too, that the Ents, these noble and ancient creatures, were doomed to die out because they cannot find their wives. What makes it worse is that the Ents do not seem to want to look and I think that the Entwives probably didn't want to be found. Maybe they just moved to the Old Forest and slowly became more treeish. Only maintaining the tiniest sparks of life.
But I remember a while back that some guy declared that he had found the Entwives within Tolkiens work and the the author had used a variety of clues to conceal where they were. Whether this was all nonsense I'm not sure but it certainly intrigued me none the less.
Ah, here we are:

http://forum.barrowdowns.com/showthread.php?t=12512

[Eönwë]

Maybe they just grow like trees. Well, what I mean is they start slightly tree-ish, but as they get older develop into more tree-like beings, depending on which tree they chose to guard.

I can just imagine it: Over time. First a little dark patch, then a spot, then a lump, then a shoot, and after about 20 years of slow growing, a small brach.

edit: A sad thought just came into my mind. Maybe all ents are sufferers of a disease, like this?