Ghidorah

Physical
Ghidorah is a large, three-headed dragon who can grow to be even bigger than Godzilla Himself. His scales are a deep golden color, and his wings are so large that their length still remains unspecified. His bio-electrical nature allows him to create large storms filled with his "gravity beams", which are essentially overcharged yellow lightning bolts that he can either emit from his body, or fire from any of his three mouths. Each of his heads possess their own brain, meaning they all act independently. The heads will often argue, until they recognize a common goal. It is still unknown if all the heads need to work together to move the body, or if one head specifically controls the body alone.

Roar
Ghidorah's roar in the Monsterverse is described by Michael Dougherty as being a modernized and revamped version of his original Showa cackles, using the same shriek-like format. Sometimes his heads roar one at a time in succession, creating different sounds. He also uses several squealing, gurgling, and growling sounds at different times, and sometimes also growls like a jaguar. At one point he uses a Heisei roar, and also uses the Showa flying sound (barely audible, right before he lands in Boston)

Personality
Each of Ghidorah's heads have different personalities, which Michael Dougherty himself says, and can also be displayed at other times in the movie. He also says their names mean "One", "Two", and "Three" in Japanese.

The middle head, "Ichi" is the master of the other two. He is the most evil and sadistic, having pure malice in his actions. It is unclear whether or not Ichi controls the body completely, or if the three heads have to work together, but Ichi is clearly in charge of his companion heads.

The right head, "Ni" is loyal to the middle head, but also the most irritable, and most aggressive. Michael Dougherty describes Ni as being "Tenacious" meaning Ni is probably the smartest of the three, while still answering to the middle head.

The left head, "San" (Nicknamed "Kevin" by the Monsterverse fanbase) is the least intelligent of the three heads, often displaying curiosity at certain times and getting distracted, much to the annoyment of the middle head, who must force his companion back on task.

The three heads have been given other names both inside and outside of the movie. Rick Stanton at one point refers to them as "Moe, Larry, and Curly", a joking reference to the three stooges. Another set of names is "Gin" (Middle Head), "Po" (Left Head), and "Ka" (Right Head)

Godzilla, King of the Monsters (Comic)
In an unspecified year, when the earth is in its earlier stages, King Ghidorah falls as a golden egg to the surface of Earth. The flash of light as he enters the atmosphere is seen by Godzilla and Anguirus, and they go to investigate, but they run into Megaguirus.

Ghidorah, the Three-Headed Monster (Comic)
In 1841, Ghidorah is attacking a group of whale hunters when Godzilla approaches him, and they begin to fight. After using an overcharge, Godzilla becomes tired, and Ghidorah moves in to kill him, but Anguirus comes in to help. Together, Godzilla and Anguirus push Ghidorah off a cliff between two icebergs, then collapse the iceberg on top of him, freezing him in ice for the next hundred years until 2019.

Godzilla: King of the Monsters (Movie)
Ghidorah will battle Godzilla, Mothra, and Rodan in the 2019 sequel to Legendary's 2014 Godzilla film.

Legendary has revealed in bits and snippets of the movie seen in the trailer that Ghidorah will have alien origins, evidenced in one shot of him falling through the earth's atmosphere with a bright glowing light.

Godzilla III (Comic)
The Xiliens approach Earth and revive King Ghidorah and Hokmuto, transforming them into Mecha-King Ghidorah and Gigan. The two of them Battle Godzilla, and are ultimately defeated.

Godzilla VS Mechagodzilla
A while after the events of Godzilla III and the Rodan standalone book, Mechagodzilla is created to assist Godzilla in fending off alien forces and other evil monsters. The humans use parts from Mecha-King Ghidorah to build Mechagodzilla, which eventually allows the Xiliens to remotely take control of Mechagodzilla, turning the human's own weapon against them. Godzilla, of course, is able to destroy the metal beast, foiling the Xilien's plans.