I can't see Tharonwe messing around with the undead. Don't know why. I see the mewlips or merlocks, whatever you want to call them, as some kind of night-wakeful cannibalistic humanoid that has developed excellent survival skills for swamp life. Sort of Gollumish without the ring-lust.
Wrong place for this, except that I brought it up in this context,

but it occurred to me that an invisible hobbit-eating Smeagol could possibly be the seed for the folklore behind the poem. Anyway....
By the by, I edited post # 484 to account for Gwyllion's nightmares of Aeron drowning.