By Shared Traits:
* Mammals: All mammals have mammary glands, fur or hair, give birth to live young, and nurse their offspring. Examples: dogs, cats, whales, humans.
* Birds: All birds have feathers, wings, lay eggs, and have beaks. Examples: eagles, penguins, hummingbirds, robins.
* Reptiles: All reptiles have scales, lay eggs, and are cold-blooded (ectothermic). Examples: snakes, lizards, crocodiles, turtles.
* Amphibians: All amphibians have smooth, moist skin, lay eggs in water, and undergo metamorphosis. Examples: frogs, toads, salamanders, newts.
* Fish: All fish have gills, fins, and live in water. Examples: sharks, goldfish, tuna, salmon.
* Insects: All insects have six legs, three body segments (head, thorax, abdomen), and often wings. Examples: ants, butterflies, beetles, bees.
By Habitat:
* Desert animals: Animals adapted to survive in dry, hot environments. Examples: camels, scorpions, rattlesnakes, roadrunners.
* Aquatic animals: Animals adapted to live in water. Examples: dolphins, sea turtles, jellyfish, starfish.
* Forest animals: Animals adapted to live in wooded areas. Examples: squirrels, deer, owls, monkeys.
By Diet:
* Herbivores: Animals that eat only plants. Examples: cows, rabbits, giraffes, elephants.
* Carnivores: Animals that eat only meat. Examples: lions, tigers, wolves, sharks.
* Omnivores: Animals that eat both plants and animals. Examples: humans, pigs, raccoons, bears.
By Behavior:
* Social animals: Animals that live in groups. Examples: wolves, ants, monkeys, dolphins.
* Solitary animals: Animals that live alone. Examples: tigers, panthers, snakes, owls.
By Evolutionary Relationships:
* Primates: Animals with a close evolutionary relationship to humans. Examples: apes, monkeys, lemurs.
* Cetaceans: Animals with a close evolutionary relationship to whales and dolphins. Examples: porpoises, narwhals, beluga whales.
To give you a more specific answer, please tell me what you're interested in! What kind of commonality are you looking for?