There are different ways that animals can be classified: Phenetic System, using physical characteristics, and Cladistic System, using evolution as a tool for grouping animals.
Cladistics looks at an evolutionary relationship between species and in order for a species to be grouped, they must share a common ancestor and be included in the same taxonomic group.
The duck-billed platypus, or ornithorhynchus anatinus, is a semi-aquatic egg-laying mammal and exists in the order monotremata, alongside four echidna species.