I think we can rule out Entwives because:
- The Entwives Gardens were to the east far from the Shire,
- In Letter 144 Tolkien said:
I think that in fact the Entwives had disappeared for good, being destroyed with their gardens in the War of the Last Alliance (Second Age 3429 – 3441) when Sauron pursued a scorched earth policy and burned their land against the advance of the Allies down the Anduin...
- Entwives don't tend to wander, as per Treebeard in The Two Towers (emphasis mine)
But our hearts did not go on growing in the same way: the Ents gave their love to things that they met in the world … for the Ents loved the great trees; and the wild woods, and the slopes of the high hills; and they drank of the mountain-streams, and ate only such fruit as the trees let fall in their path; and they learned of the Elves and spoke with the Trees.
But the Entwives gave their minds to the lesser trees, and to the meads in the sunshine beyond the feet of the forests; and they saw the sloe in the thicket, and the wild apple and the cherry blossoming in spring, and the green herbs in the waterlands in summer, and the seeding grasses in the autumn fields …
So the Entwives made gardens to live in. But we Ents went on wandering, and we only came to the gardens now and again*
The Lord of The Rings - The Two Towers - Treebeard
So it would be unlikely to have been one of the Entwives.
Unlikely to be an Ent
The Ents all seemed to be located in Fangorn Forest and when Merry and Pippin are talking about the Shire to Treebeard in the Two Towers
He made them describe the Shire and its country over and over again.
He said an odd thing at this point. 'You never see any, hm, any Ents round there do you?' he asked. 'Well, not Ents, Entwives I should really say.'
'Entwives?' said Pippin. 'Are they like you at all?'
'Yes, hm, well no: I do not really know now,' said Treebeard thoughtfully. 'But they would like your country, so I just wondered.'
The Lord of The Rings - The Two Towers - Treebeard
Suggests the Ents haven't been to the Shire
and the following from The Two Towers
For many years we used to go out every now and again and look for the Entwives, walking far and wide and calling them by their beautiful names. But as time passed we went more seldom and wandered less far. And now the Entwives are only a memory for us, and our beards are long and grey
Says that the Ents have stopped looking for the Entwives and had even stopped leaving Fangorn at all.
Maybe a Huorn?
So we are left with possibly a Huon left over in the old forest?
But Huorns were more tree shaped than man shaped, and wrapped in shadow - so unlikely. But some theories suggest that Huorns may in part be devolved Ents, so might retain some man shape in some cases.
Or a Giant / Ogre
Where the only way Hal had of describing it was "as tall as an Elm"?
Suggesting it might be a giant or ogre can be inferred by some of Tolkien's Letters where he admits that the Ents were not invented until he came to Treebeards chapter.
I did not consciously invent them [the Ents] at all. The chapter called 'Treebeard', from Treebeard's first remark on p. 66, was written off more or less as it stands, with an effect on my self (except for labour pains) almost like reading someone else's work.
Letter 163
And I like Ents now because they do not seem to have anything to do with me. I daresay something had been going on in the 'unconscious' for some time, and that accounts for my feeling throughout, especially when stuck, that I was not inventing but reporting (imperfectly) and had at times to wait till 'what really happened' came through.
Letter 180
I have long ceased to invent … I wait till I seem to know what really happened. Or till it writes itself. Thus, though I knew for years that Frodo would run into a tree-adventure somewhere far down the Great River, I have no recollection of inventing Ents. I came at last to the point, and wrote the 'Treebeard' chapter without any recollection of any previous thought: just as it now is. And then I saw that, of course, it had not happened to Frodo at all.
There are or were no Ents in the older stories – because the Ents in fact only presented themselves to my sight, without premeditation or any previous conscious knowledge, when I came to Chapter IV of Book Three [the ‘Treebeard’ chapter].
Letter 247