A fish becoming (or even giving birth to) an elephant would be evidence against evolution not to prove it. Evolution is based on the idea (and observed fact) that organisms (even ones that undergo metamorphosis like butterflies and frogs) never actually change species during their lifetimes, and that the immediate descendants of any individual differ only slightly from their parents. It is only over many generations that small differences between distant cousins may accumulate to the extent that they are no longer mutually fertile and so form different species.
But although no fish ever became an elephant, there is a clear line of fossil records showing how (over almost half a billion years – which may be a bit longer than you have actually been looking) descendants of some fish became more and more like what we call tetrapods, which had descendants that were amphibians, some of which had descendants that were reptiles, some of which had descendants that were birds and others of which had descendants that were mammals, and some of these mammals were elephants.
The beginning of this story is that the first land vertebrates, or tetrapods, evolved from lobe-finned fish (sarcopterygians) approximately 375–390 million years ago during the Devonian period. These transitional creatures, such as Tiktaalik, Acanthostega, and Ichthyostega, developed lungs and robust limbs to navigate shallow, swampy, and, eventually, terrestrial environments.
Key Early Land Vertebrates & Evolution
- Transitional Forms: Tiktaalik is a crucial fossil exhibiting a mix of fish and tetrapod features, including limbs with bones, yet still possessing gills and scales.
- Early Tetrapods: Acanthostega and Ichthyostega are among the earliest, most complete fossilized tetrapods, showing clear, specialized limbs for movement on land.
- Adaptation Timing: While early tetrapods appeared around 375 Ma, they likely spent significant time in shallow water before becoming fully terrestrial.
- Evolutionary Significance: These animals descended from aquatic ancestors and eventually diversified into all modern amphibians, reptiles, mammals, and birds.
- Dietary Shift: Early land vertebrates were primarily carnivores, but researchers have recently found evidence of early plant-eaters, such as Tyrannoroter heberti, by the Carboniferous period.
But the rest of it is far too long to include here though there are plenty of very complete versions that you could easily find if you looked.
Of course you probably knew that already and are just trolling. But perhaps this answer may be useful to someone less stupid who is looking for the right words to explain things to a nine year old.