Mammals are the only animals that have hair. Would you consider the presence of

The presence of hair in mammals is said to be a derived trait among all

animals.

Mammals generally have hairs which is one of their major

characteristics.Animals such as amphibians and reptiles were in existence

before mammals and they had no hair present on their body.

As time went on, changes started to occur as a means to adapt to the living

conditions which gave rise to the development of hairs in mammals and is

the reason why it is a derived trait.

Read more about Mammals here link

derived trait

Explanation:

An ancestral (or primitive) trait/character is a feature inherited from the common ancestor of the species/group of interest, while derived traits are those that were absent in the last common ancestor of the group of interest. For example, considering mammals as the target group, the presence of hair is a shared derived trait relative to other vertebrates, i.e., amphibians, reptiles and birds. However, this trait (hair) is ancestral for humans since the genetically closest species to humans (e.g., chimpanzees), also have hair, and they have inherited this trait from a common ancestor.