Intellect Devourer

Dog-sized brains with stumpy boney legs, these strange creatures are far more deadly than they first seem, possessing psionic abilities to eat the brains of other creatures, and replace them. In this way an Intellect Devourer can live inside someone's skull, and use their memories to pretend to be that person. Notable victims of this in Waterdeep are Meloon Wardragon and Floon Blagmar.
They seem to find home within the sewers, with some affiliation for the creature known as Nihiloor.