A Moomintroll is a creation of Finnish author Tove Jansson, and the title character of a series of books that she wrote for children. It's a modern fairytale, really; set in fantasy forests of Finland (oo, that scans!). As to what, precisely, a Moomintroll is, I present:

"A Moomintroll is small and shy and fat, and has a Moominpappa and a Moominmamma. Moomins live in the forests of Finland. They like sunshine and sleep right through the winter. The snow falls and falls and falls where they live, until their houses look like great snowballs. But when spring comes, up they jump...."
- Kaye Webb, editor of Puffin Books 1974.

I strongly recommend these books for grade-school readers. This information comes from the following webpage, which I recommend you see for more info on the books: http://www.io.com/~fazia/Moomin.html