Here's my list. If you got them all "right" you have a very good understanding of animal classification (probably learned outside this course).

Insects (have "exoskeletons" with six jointed legs):
butterflies
mosquitoes

Birds (have feathers):
hawks

Mammals (have hair - although some have more hair than others):
bats
cattle
dogs
dolphins
elephant
mice

Fish (have scales and gills):
cod
goldfish
trout

Each of these groups represent animals related by ancestry and I have include the homologous similarity they share in parenthesis.

Notice that some of these homologous structures may help with the animals' lifestyle but are not the only way to tackle the lifestyle. For example, the homologous structures of fish (scales and gills) are beneficial to an animal that lives in water but dolphins have evolved different ways to live in water without the need for gills or scales.

Notice that bats and dolphins are now in the same group in spite of the fact that they look very different from the other mammals. But remember, those differences are due to the extremely different lifestyles of the animals. This classification is based upon similarities due to ancestry and those similarities can be very subtle. Elephants and dolphins have very little hair, but what little hair they have is the "baggage" they inherited from their ancestors. Dolphins and elephants can get along without the hair, so their hair is nothing more than old junk handed down through the generations. If a designer were responsible for the evolution of dolphins or elephants, why would he have given them any hair at all? The theory of Natural Theology doesn't make sense of the hair. Lamarck's ideas about the Inheritance of Acquired Characteristics cannot explain the hair on elephants or dolphins either. The presence of this shared characteristic, hair, is best explained by all mammals having shared a common ancestor. Homologous similarities are a fine example of modification by descent. Hair is a homologous similarity that unites all mammals whether they put it to good use or not.

Sometimes homologous characteristics are retained but used for a different lifestyle. The feathers of birds help them fly but even birds that do not fly still have feathers. Penguins don't fly but their feathers provide important insulation so they are useful to the birds' lifestyle and used in a different way. Regardless, penguins are birds because they have feathers.

As you can see, homologous similarities are more difficult to follow but they provide important clues to ancestry and provide explanations as to why they are there at all. Classifications based upon homologous similarities, instead of analogous similarities, allow us to focus on the evolution of the organisms and appreciate their ancestry.

Here's a wee puzzle that may help focus your mind on homologous similarities. I'll admit, this is not easy, but you'll learn a little from it.
Which animal is more closely related to a goldfish - a man or a shark?