I have another take on this.
An Ent is not a tree, it's a giant. It's a tree shepherd, a being brought into Arda to protect the trees which can very much have the appearance of trees, and rather like a master may come to look a bit like his dog, an Ent can come to look like the trees he shepherds.
Ent is also the Old English word for 'giant', with a concurrent word in Old Norse -
jotunn. Giant lore is something I've not looked into agreat deal because I find them frankly a bit scary (don't laugh
) and unappealing, but Ents as giants I find an appealing idea.
I think that as they age, they become more like a tree and might become as static as a tree so that to the untrained eye, they
are a tree. Some of these Ents may, I suppose be termed 'Huorns', though that's highly debatable.
There's something interesting that was posted a few years back (and I've been trying to find this) about how a 'houseless Elf', that is one who has been killed, may choose not to go to the Halls of Mandos and instead remain a houseless fea and maybe inhabit elements of the earth itself, such as rocks and trees. I think that is also a possibility for some Huorns. And for others, that they are simply trees that are capable, if pushed, of movement and speech.