Strictly speaking, the smallest animal in the world would be an amoeba. Amoebas are unicellular organisms, which means they are comprised of only a single cell and are classed in the animal kingdom under Protozoa. So, technically speaking, Amoebas are actually animal-like protists classifed in the kingdom Protista, and thus not animals.
However many people do not consider these to be "living creatures", despite their inclusion in the animal kingdom. The other smallest animals, in order of there size, are:
The smallest insects are the male parasitic wasps. They measure only 139 micrometers in length. The females are around 40% larger than the males. These are the smallest animals, when amoeba and the like are not included.
The smallest beetle belong to the Nanosellini tribe. They all measure less that 1mm in length. The smallest in Scydosella musawasensis at 300 micrometers long.
The smallest animal with a backbone is a species of fish, known as Stout Infant Fish. It measures just about 7mm in length and is found in Australia's Great Barrier Reef.
The smallest amphibians are the Monte Iberia Eleuth and the Brazilian Gold Frog. Both measure around 9.6mm to 9.8mm in length.
The smallest reptile is the Jaragua Sphaero, also called Dwarf Gecko. It measures 16mm long.
The smallest bird is Bee Hummingbird, as the measure around 5 cm in length.
The smallest mammal is Kitti's Hog-nosed Bat, also known as the
Bumblebee Bat. It is found in Thailand and it only measures 30mm to
bird : humming bird
insect : ant
The smallest animal would be the ameba.
And if you are referring to the smallest "mammal", the correct answer would be The Shrew.