Leviathan is a large creature found within Equus, and is the queen of her own species that fought against Yellow Diamond and Rhodonite.
Physical Appearance
Compared to her subjects, Leviathan is exceptionally large, tall enough to match Yellow Diamond's height. She has two strong arms with webbed hands, a set of upward-facing tusks, and reddish skin compared to the more common blue of her subjects. She has a "Gem-like" face.
She is considered to be very disgusting and ugly by most other people, being given the nickname "Queen of the Ugly Nasty Creatures".
Personality
Leviathan is capable of speech, but can only speak in very broken English. She is very brutish towards both her subjects and intruders, calling many beings smaller than her as insects.
History
According to legend, Leviathan emerged from the dark abyss of the sea with intent to take over the world. She was so ruthless and aggressive, that she drove out all the Seaponies, Sirens, and other races from the oceans and onto land, taking full control of the sea. She intended to take over the land as well, but was stopped and defeated by Starswirl the Bearded. Not wanting her to rule over land or sea, she was left on the beach where she was defeated, locked away in a cave.
According to a legend told by Skystar, Leviathan tried to attack both Seaquestria and Mount Aris in a combined attack alongside Ziz, but she was forced out when the "Angels" arrived and fought them both.
While more hinted upon, it's implied she escaped her prison after the Everstorm passed through, but remained restricted to the beach.
My Little Universe II
Leviathan was later confronted along her beach by Yellow Diamond and Rhodonite, her demanding they leave her beach. She fought with Yellow Diamond directly, but quickly felt outmatched at first, retreating to the ocean briefly after the first fight. However, this was a trick, as she looped back and attacked again, managing to even the odds by cracking Yellow Diamond's gemstone, and brutally beating her up afterwards. That night, she found Lavi and Button trying to rescue Yellow Pearl, confronting them all and sending her subjects after them to capture them again. However, she found Jasper and tried beating her. However, she couldn't exclaiming how her defeat was happening all over again before being knocked unconscious.
She reappears as part of the many creatures fighting for Grogar's Army in the three-part finale, specifically targeting Yellow Diamond during the fight in the Realm of Corners. She also fights them while on Grogar's warships, but after being knocked out by Rhodonite hasn't been seen since.