The Cactuses are an anthropomorphic balloon family living in the city of Frosty Star. They all have the appearance of an anthropomorphic balloon, with varying sizes and shapes, tall and small. As well as resembling balloons, they all have pen faces of varying sizes. The fact that their bodies have strings in them suggest that it is a type of balloon.
As Toby was revealed to be a cactus in "The Shell," it can be assumed that the rest of his family are also similar beings.