This is not a creature of its own, instead it is a classification
Dwarf creatures can occur in any TF2-nimal, and is an uncommon occurence. The effected organism is perfectly shrunken, but functions exactly the same if it was normal-sized. There are two types of dwarf creature:
The Mutation Dwarf
The Natural Dwarf
The Mutation Dwarf occurs when the genetics of one creature is effected so that it's growth is stunted permanently. This change happens usually when the organism is 1-5 months old. The Natural Dwarf however, is born like this. Their parents are also usually dwarfs, so the genes are passed down from generation to generation. The first generation would have probably been a Mutation Dwarf, but after many years of breeding, it had been classified as Natural.