In order to make every type of creature viable, you may upgrade your pets to your "Animal Taming tier" in order to increase its statistics and damage.
If you meet the requirement, you will be able to upgrade your creature for the following:
(Your taming skill - Your creature taming requirement)%
Which means that if you are a legendary tamer (120) and decide to upgrade your nightmare (100), your nightmare will evolve and gain 20% statistics and damage.
To upgrade your creature, simply click on it once and use the "Upgrade Creature" option.