Grindylows

Grindylows are a mythical species mentioned in The Menagerie trilogy.

The Menagerie
When Robert Kahn mentions Form B7504 to Melissa Merevy, she interprets it as someone being eaten by a grindylow.

Trivia

 * Grindylows are one of the only creatures not described in Camp Underpaw's Guide to Mythical Creatures, along with barangs, bunyips, elves, enormous talking cockroaches, monster spiders, tooth faeries, vampires, and will-o'-the-wisps.