That is just a lack of basic understanding of how evolution works. It is not something that can happen where a dog turns into a cat. Evolution occurs very gradually over several generations. However, in organisms with short lifespans, you can see evolution occur relatively quickly. For example, moths. Before the industrial revolution, lighter colored moths were more prevalent than darker colored moths as they could blend in with the lighter colors of the wood. However, after pollution and smog, the environment was easier to blend in with if you were a darker colored moth. So the darker colored moths were the ones who were able to survive because predators couldn't see them and reproduced. Eventually, the darker colored moths had a higher population than the lighter colored moths in urban areas, while in the countryside the lighter colored moths were more populous due to the lack of smog.
In a similar manner to this, that is how animals are able to evolve. However, there is the requirement of the potential to evolve in that manner. There were already light and dark moths, so the environment naturally selected for which ever color it favored. If you put the moths in a purple environment, they wouldn't turn purple because they never had the ability to turn purple, or any genes that would turn them purple. There must also be a variation of a trait that the environment naturally selects for/against. Another example of this is giraffes. Shorter necked giraffes were less likely to survive because they couldn't reach the leaves that were higher up, so the pressure of drought/food competitor meant that the longer necked giraffes were able to survive. the giraffes with slightly longer necks were able to live to reproduce and passed on the genes that made a longer neck possible. So gradually over time, giraffes grew longer necks.
A similar but opposite effect is happening with elephant tusks. Elephant tusks vary in length depending on what genes an elephant has, so there are longer and shorter tusks. Elephants with long tusks are more desirable to ivory poachers, so the elephants with the long tusk genes are killed. This means that the shorter tusked elephants are the ones who are surviving and passing on their genes for shorter tusks. Prior to poaching for ivory, longer tusks were selected for because their tusks helped them fight other males for mates, and it was easier to win if you had longer tusks, so the long tusked elephants passed on their traits. Depending on the environmental pressure applied, the tendency towards shorter or longer tusks also changed.
You should take a closer look at other evolutionary examples that look into the smaller details of certain species gaining adaptations to suit their environment, and then broaden out that focus to the evolution of an aquatic animal into a land animal. Having a grasp on how smaller traits can change might help you understand how it is that major changes in a species' physiology can and have occurred over millions and millions of years.
That being said, it's all quite a clever process, who are humans to say that evolution as a process wasn't implemented by a higher being with the intentions to give each species a fighting chance at survival?