The Mewlips

[Encaitare]

In "The Tolkien Reader" are a bunch of poems, supposed to have been taken from the Red Book (I believe). My favorite of these is called "The Mewlips":

The shadows where the Mewlips dwell
Are dark and wet as ink,
And slow and softly rings their bell,
As in the slime you sink.

You sink into the slime, who dare
To knock upon their door,
While down the grinning gargoyles stare
And noisome waters pour.

Beside the rotting river-strand
The drooping willows weep,
And gloomily the gorcrows stand
Croaking in their sleep.

Over the Merlock Mountains a long and weary way,
In a mouldy valley where the trees are grey,
By a dark pool's borders without wind or tide,
Moonless and sunless, the Mewlips hide.

The cellars where the Mewlips sit
Are deep and dank and cold
With single sickly candle lit;
And there they count their gold.

Their walls are wet, their ceilings drip;
Their feet upon the floor
Go softly with a squish-flap-flip,
As they sidle to the door.

They peep out slyly; through a crack
Their feeling fingers creep,
And when they've finished, in a sack
Your bones they take to keep.

Beyond the Merlock Mountains, a long and lonely road
Through the spider-shadows and the marsh of Tode,
And through the wood of hanging trees and the gallows-weed,
You go to find the Mewlips - and the Mewlips feed.

I was intrigued by this poem, which is very different from the other ones in the Reader, and from anything I have ever read by Tolkien -- much darker and grimmer, and it rather creeped me out. Since I had never heard of these Mewlip creatures before, I searched to find out anything I could about them. This was all I could find:

According to the lore of hobbits, an evil race of cannibal spirits called the mewlips settled in certain marshlands of Middle-earth. Hoarding phantoms very like the dreaded barrow-wights they seemed, but they made their homes in foul and dank swamps. Travellers in their lands always walked in peril, for many were said to be waylaid by these beings.

Does anyone know anything more about them, or if Tolkien ever mentioned them elsewhere? The style of the poem seems rather un-hobbitlike, as I said, a bit too dark for the usually jovial and light-hearted people. Perhaps Tolkien was in a rather dark mood or time when he wrote it?

Any help or discussion is appreciated!