Kergrim were horrible creatures of the Underdeeps. Vaguely amphibian-Troll- or giant-Frog-like they were often guessed to have been undead descendants of Troll-kind that had been magically warped by the Mordo or Unlife. Kergrim always appeared where there were ancient Tombs and relics of the dead, and they were believed to either feed from the dead or to be attracted by the vapour of massgraves and cadavres.
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The Kergrim does not appear in the Works of Jrr Tolkien.Instead it is taken from the folklore of some parts of england and scotland where "Kergrim" refers to a type of Ghoul or Wraith that haunts gravesites or churchyards.Alternatively the name may come from a breton placename, Ker being a word for "fortress" and grim possibly being a germanic loanword meaning "roaring, angry, ugly or nasty".
It has also been suggested that the Kergrim may have been inspired by Tolkien's Mewlips poem.