Rokmutul concept art was submitted by both Josh Nizzi and Frank Hong early into the development of Godzilla.
Rokmutul resembles Anguirus or an Ankylosaurus very closely, with the spikes running down its back and its armory appearance. It has a streamlined crocodilian body, and spikes on its tail closely resembling those of a stegosaurus. It also has six legs, making it resemble the Unnamed Multi-Legged Monster, though that monster had at least eight legs.
The monster's history is unknown, though from concept art it would appear Rokmutul would've fought Godzilla atop Alcatraz in San Francisco. Its relation to the other unmade Godzilla monster, Pterodactyl, is unknown.