If Evolution is True, Why Don’t Dogs Turn Into Cats?
In a nutshell, evolution makes no such prediction. In fact it says the opposite; it cannot happen.
If a dog did suddenly turn into a cat, I would consult a magician not an evolutionist. Everything in evolution is glacially slow. If dogs turned into cats over a million generations, that would be evidence evolution is wrong. At least for mammals, the tree of life has no cross connections, e.g . from the canine branch to the feline. All cats evolve from feline ancestors. Dogs and cats have a common ancestor. The tree diverges into felines and canines. It never reconverges by dogs evolving back into their common ancestor or a cat.
Further cats and dogs have evolved for quite different environments and lifestyles. They are always getting slightly fitter for those two different environments, in other words more different. Evolution is not random as the questioner imagines. Natural selection ensures evolution does not run in reverse toward more similar but less fit animals.
On the other hand, convergent evolution sometimes pushes wildly different animals such as sharks and dolphins toward the same external shape, even though they have completely different innards, because they live in the same environment.
The question indicates the questioner has not yet had any exposure to a scientific education. They have heard only straw men descriptions of evolution. If you are serious about what evolutionists really have to say, see Evolving: The Human Effect and Why It Matters by Dr. Daniel J. Fairbanks. You will learn that evolution is a fact, observed in the body, in lakes, in fossils and in the test tube. You will learn how DNA (Deoxyribonucleic Acid) mediates the whole process. You will learn about chromosome splitting and fusing. You will learn about the types of error DNA makes while copying itself. These errors are the source of variation in future generations.
Evolution is also a theory, in the same sense as music theory or the theory of gravity, not in the colloquial sense of just a hunch, but in the scientific sense of a precise mathematical explanation of how and why animals and plants gradually change over time.~ Roedy (1948-02-04 age:70)