Kaiju

Kaiju are huge amphibious monsters that make their appearance in Big Hero 6: The Animated Series. They are creatures of unknown origin. They are widely feared by all of humankind for the destruction they cause when they make landfall in major cities.

History
The Kaiju are a race of gigantic amphibious creatures that are typically around 200-300 feet tall. They are toxic and extremely hostile creatures with skin that even the mightiest military weapons cannot penetrate. It's not known where exactly these creatures came from or how they came to be, but we do know they first made themselves known to humanity in the 1950's, during the Atomic Age, when a Kaiju was first spotted swimming in the ocean by submarines. The first Kaiju attack on the city came about a few months later, where a Kaiju rose up and devastated the city of Osaka. Since then, scientists across the world have been trying to study these behemoths to learn more about their abilities, how to prepare people for the next attack, and how to maybe even stop them completely. The Kaiju attacks on the world were the reason why Wasabi built Red Ronin, a giant robot fighting machine built to fend off the beasts.

The deadliest recorded Kaiju attack in history took place in San Fransokyo in 2022, and it was dubbed, "The Great Siege" by  where an enromous 330 foot tall Kaiju, the largest on record, came out of the ocean and began to wreak havoc. Despite the best efforts from the military to stop it, the Kaiju devastated the city, killing millions and creating a 40 mile path of destruction from San Fransokyo to Los Angeles. This particular Kaiju was responsible for the deaths of the parents of Hiro Hamada andTadashi Hamada. Tadashi was only a boy at the time of the attack, and Hiro was just a baby.