About the game
Crittermino is Fillomino played with animal homes. The rules are older than this site. The animals are how the sizes stay easy to remember.
Where Fillomino comes from
Fillomino first appeared in 1994, in volume 47 of Puzzle Communication Nikoli. A reader writing under the name すらんた (Suranta) suggested it. The Japanese name フィルオミノ joins the English words “fill” and “polyomino”: you fill the grid with polyominoes, and every cell of a piece shows how many cells that piece has.
It grew out of an earlier Nikoli idea, Firumatto, where the pieces were straight 1×n rectangles and pieces of the same length could not touch. Fillomino keeps the “same size stays apart” rule and lets the shape bend, as long as the cells share sides.
That history is written up by Grandmaster Puzzles and the World Puzzle Championship wiki. The genre also has a page on Puzzles Wiki.
Why the numbers became animals
In a printed Fillomino, a 4 means “this region has four cells.” Crittermino uses a dog for that same fact. Mouse is 2, cat is 3, dog is 4, panda is 5, and lion is 6. You still fill every cell. A home still has to be the right size and connected edge to edge. Two homes of the same size still cannot share a wall.
The campaign has 300 levels. The same level number is the same puzzle for everyone, and each one has a single solution. Levels 1 to 5 teach the rules. After that the grids grow from 5×5 to 7×7. Progress stays on your device. There is no account. The daily challenge is one of those campaign grids, shared for the UTC day, and clearing it does not skip you ahead.