Seven Kingdoms: Seowyn's Crossing
Kumaats are terrible Fey creatures of nightmare. They delight in manifesting among “weaker” mortal species on Tol and forging legends of “bogeymen” and hauntings. They do this by entering a normal mortal animal and turning it into something huge, twisted, and intelligent, then preying on the mortal around it. If it’s killed, its spirit flees to the Feywild, and, later, it simply moves its spirit into another mortal animal, thus giving rise to the idea that the creature is an immortal terror.
The most recently reported example of a kumaat is that of Limba, a terrible crocodile that menaced the bayous of Dalenshire, becoming a legend amongst halflings. Eventually, Tilly Thistleshanks and the Fire Wasps slew the creature, pursued it to the Feywild and slew it there, capturing its disembodied spirit and giving it to the keeping of Granmere Odetta.