Which one of the five is least like the other four?

Cow, Tiger, Dog and Bear are all mammals but Snake is not a mammal but a reptile.

A mammal is any vertebrate animal that are known to have mammary gland and feed their young ones with milk produced from the glands. Except for whales, the presence of hair on the skin is a typical characteristic of a mammal. Most mammals also give birth to their young ones alive. These characteristics distinguish the mammals from the birds and reptiles.

On the other hand, reptiles are vertebrate that are covered in special skin made up of scales or bony plates. They do not maintain a constant body temperature. Most of them reproduce by laying eggs. Their young ones can glide, swim and walk within hours of birth.