The other day on episode 193of the The Prancing Pony Podcast Alan and Shawn were discussing Treebeard's statement to Merry and Pippin in TT 3.4.
There are Ents and Ents, you know; or there are Ents and things that look like Ents but ain't, as you might say.
I think there's a bit more wordplay going on here than the simple charming slant rhyme of 'Ents but ain't'. Paradoxically, I caught the wordplay because of Philip Pullman, well known for being no fan of Tolkien. In chapter 7 of The Golden Compass, for example, Lyra says:
'I en't never deceived anyone!'
Lyra uses 'en't' for 'ain't repeatedly, as do other characters. Even without an electronic copy of the text, examples abound. According to the OED, 'en't' and 'ent' are but two of many regional and nonstandard variations on 'ain't'. Lyra is of course also a native of Oxford, brought up in one of its many colleges, but her world is not quite ours. So there, 'en't' seems more common than here.
But it's common enough here for Tolkien to pun on it.
(I just wanted to dash off a quick post here. I would welcome any further information on the use of 'ent' and en't', particularly around Oxford.