Sanda is the name given to one of two genetic offspring of "Frankenstein." During his final battle with Baragon, Frankenstein lost some skin tissue. Due to his unique genetic makeup, these stray cells soon grew into an entirely new being, the "gargantua" named Sanda. Initially a human-sized, furry creature, Sanda was found by some hikers and brought to the laboratory of Dr. Paul Stewart and Dr. Yuzo Majida. The two, having already obtained some of the original Frankenstein cells through Dr. Kita, opted to continue Dr. Bowen's work and kept Sanda with them to study him.
Sanda proved to be a very gentle and kindhearted creature, taking to the humans quite well, especially Akemi the lab assistant. Eventually, though, curiousity got the better of Sanda and he escaped from the lab, as his "father" had before him, and fled to the mountains where he lived for five years. He quickly grew to giant size. Despite his immense height, his gentle demeanor and affection for humans remained, and Sanda deliberately kept to himself and stayed away from humans for fear of frightening them.
Five years later Sanda discovered quite by accident that he had a "brother" in the form of Gaira, who was exactly like him, but green. Sanda found Gaira being attacked by the Japanese Self-Defense Force and, unaware of Gaira's violent nature (or that he himself had until now been blamed for Gaira's crimes), Sanda came to his sibling's rescue and took him back to his mountain home and nursed him back to health. But Sanda quickly learned of Gaira's true nature, when he returned one day to find that Gaira had killed and eaten some hikers. Enraged, he attacked Gaira.
Gaira fought his brother off and fled to the sea. Later, when Gaira came ashore once more to attack Tokyo, Sanda once again appeared, only this time he did not want to save Gaira, but to kill him. The two huge monsters fought, destroying much of Tokyo, before their battle took them out into the open ocean. There, a suddenly appearing undersea volcano erupted, killing both Sanda and Gaira even as they were locked in mortal combat.
In the English dub of War of the Gargantuas, Sanda is simply called the Brown Gargantua.