"The smallest animal" in Engpsh can refer to the animal with the smallest physical size or the animal that belongs to the smallest species or taxonomic group. Here are some examples with Chinese translations:

"The smallest mammal in the world is the bumblebee bat, also known as the Kitti's hog-nosed bat." (世界上最小的哺乳动物是蝙蝠，也称为Kitti的猪鼻蝠。)

"The pygmy marmoset, native to the rainforests of South America, is one of the smallest primates in the world." (熊猴，原产于南美洲的热带雨林，是世界上最小的灵长类动物之一。)

"The tardigrade, also known as the water bear, is a tiny animal that can survive extreme conditions such as extreme cold and heat, and even the vacuum of space." (水熊，也称为细菌熊，是一种能在极端条件下生存的小动物，如极寒和极热，甚至太空真空。)

In general, the term "small" is used to describe physical size, while "tiny" is used to emphasize how small something is. "Minuscule" is a formal term that means extremely small.