Special effects artist Patrick Tatopoulos was contacted by director Roland Emmerich and asked to create a new design for Godzilla. According to Tatopoulos, the only specific instructions Emmerich gave him was that it should be able to run incredibly fast. Godzilla was originally conceived by Toho special effects directors Eiji Tsubaraya and Akira Watanabe as a robust, erect-standing, plantigrade reptilian sea monster and played by an actor in a monster costume; Tatopoulos reimagined it as a lean, digitigrade bipedal iguana that stood with its back and tail parallel to the ground and rendered via computer animation. The monster’s distinctive facial features include a prominent lantern jaw, inspired by the fictional tiger Shere Khan from Disney’s animated adaptation of The Jungle Book. Zilla’s color scheme was designed to reflect and blend in with the urban environment. At one point, it was planned to use motion capture to create the movements of the computer-generated monster, but it ended up looking too much like a man in a suit. Upon pending approval for the design, at the time, Shogo Tomiyama commented on Zilla's look, "It was so different we realized we couldn't make small adjustments. That left the major question of whether to approve it or not." Though the monster is referred to by the characters as a "he," Patrick Tatopoulis stated on a DVD audio commentary the effects crew sculpted female genitalia into Zilla's CG model.
Zilla is portrayed in the films as a territorial, piscivorous, 60-90 m (197–295 ft) tall, 500 ton mutated lizard. Atypical of Toho’s giant monster characters, Zilla is not immune to conventional weaponry, and instead relies on its cunning and athleticism to outflank its enemies. It can travel long distances over land and sea, burrow underground, and reproduce via parthenogenesis. It used a flammable sonic "power breath" in the film, but could breathe a green atomic flame in the animated series, in which it was pitted against a rogues gallery of original monsters such as "El Gusano" and "The Crackler", after the producers were unable to secure the rights to adapt Toho's classic monsters.