They're actually an intelligent civilization that lives in taigas. They seem to expand slowly, however, probably due to their peacefulness. In my first world they only had a small corner of one continent; I ought to check Legends to see if they were wiped out.
Strangely, they seem to use the language files of every civilization.
EDIT: There's something weird going on in this trip through legends. Moomins must not breed, or something, because I've got these moomins in the year 24 or the year 36 "of unknown parentage". Also, apparently there's only my starting seven...er, wait, no, we've got moomin law-givers, and they do seem to breed. There was also a moomin vampire named Otez Elsmotsmosuk. He was one of the first of his kind, and married Gadu Toastswept in the year 5. After spending some years wandering the wilds and getting into fights with bronze colossi, he settled in Sistersbudded, the capital of the moomin kingdom, and became its law-giver, which he still is. He then spent some years taking trips into the wilds, successfully taming leopards, jaguars and one-humped camels. In the year 69, he decided to worship the bronze colossus he had been attacked by thirty years prior. Eventually, his power caught up to him after he profaned a temple in Sistersbudded and was cursed to wander the town in search of blood. His last action was to law a series of oppressive edicts upon the moomins of Sistersbudded. He had ten children, including a son who became a "gloom freak spouse", outliving all of them except the gloom freak spouse and a daughter who became obsessed with the secrets of life and death in 57 (but isn't listed as a necromancer), many of them only living four or five years. Possibly they were included in the one hundred and six moomins he murdered? Maybe not; the gloom freak spouse carried off at least his younger sister when she was 4, after that younger sister had escaped unscathed from three encounters with his wife the original gloom freak. In fact, all of these children seem to have died at the hand of either their creepy older brother or their sister-in-law. The gloom freak spouse, followed in dad's footsteps, although feebly, killing three moomins and a dwarf. He's still around, too (the current year is 100), and has one daughter, also a gloom freak, and she had one daughter, but that great-granddaughter of the moomin vampire died in her first battle with a dwarf.
Man, them's some bloodthirsty moomins. We need to get a necromancer to raise up Tove Jansson to write Finn Family Moominvampire, which should read like a cross between the original Moomins and a family history from A Song of Ice and Fire.