Pets are animals which are kept for entertainment and companionship, rather than food or resources. Pets are impractical for an outpost or initial settlements on Mars, since they consume resources without contributing to the survival of the crew. Established settlements may wish to introduce pets in order to improve morale. First experience has been made already with Pixel on the Mars Desert Research Station on Earth.
Insects are a likely first choice as pets, since many species are easily maintained, and consume few resources.
Some species of bird may be kept in small cages. They are impractical for an initial settlement.
Mammals are unlikely to be chosen as pets in very small outposts, as their care is resource intensive. Nonetheless, mammals such as cats and dogs are capable of meeting some of the social needs of humans. Other candidate mammals include rodents and rabbits. Large settlements are very likely to have pets, and children in particular are likely to demand them.