The Hattifatteners are tall, thin, ghost-like creatures, resembling long white socks. They have round neckless heads with two round eyes. Below their heads on either side are five finger-like projections. They are silent and serious, having neither the ability to talk nor to hear, but in contrast, their sense of feeling is extremely accurate, and they can sense even the most minor tremblings of the ground. They also seem to be melancholic characters. However despite physiologically resembling animals, Hattifatteners grow from seeds. Planting Hattifattener seeds where someone has taken up residence is an effective way to get rid of him/her.
Hattifatteners travel the earth in small boats, meeting every year on a lonely island. Their main interest is in the weather and they collectively own a barometer. They seem to have an interest in lightning storms, and such conditions electrify them making them give out a pale glow, and making them dangerous to touch.
The Hattifatteners are very serious and zealous about their barometer. In Finn Family Moomintroll, a Hemulen steals their barometer, and this caused them to relentlessly pursue him until they got it back.
Some think the Hattifatteners are wicked, but this is due to their strangeness. In the story The Secret of the Hattifatteners that appears in Tales from Moominvalley Moominpappa comes to understand the mysterious Hattifatteners and their strange attraction to lightning storms. Unusually for what is ostensibly a children's book the story deals with what appears to be Moominpappa's mid-life crisis. His quest to seek out and understand the mysterious Hattifatteners is his search for a deeper meaning in life.
The Hattifatteners (the original Swedish name Hattifnattar, the Finnish name Hattivatit) are creatures in the Moomin books by Tove Jansson. Their main appearance is in the book Finn Family Moomintroll.