The Giant Mosasaur's concept art shows a generic mosasaur the size of Kamoebas with a smooth, white underbody and a darker-colored pattern on the top of its body. It had a row of sharp teeth, as well as large, crocodilian jaws, and four fins.
The Giant Mosasaur would have washed ashore, having been killed by Godzilla. The carcass would be investigated by humans and it would be determined that Godzilla had killed it.
The Giant Mosasaur was a concept that was created to replace the concept of Anguirus being the dead monster that washed ashore. However, the Giant Mosasaur was scrapped due to Toho not wanting to introduce a new monster just to have it be dead from the start, so it was replaced with Kamoebas, whom wasn't as popular as Anguirus.