The Ancient King (古代王, Kodaiō) was the king of the Terror Lizard Clan, he is also an anime only monster.
The Ancient King awakens after 300,000,000 years of slumber, which was used to wait for the other kings to be taken out. He is attacked by military forces, but the missiles are proven ineffective against the Terror Lizard's skin. The Hero Association deploy Tatsumaki to take care of the threat
After a briefly conversation with Tatsumaki, he's killed by her, after getting smashed by a meteor brought from space by Tatsumaki with the use of her telekinetic powers.
Powers and Stats
Tier: At least 7-A
Name: Ancient King
Origin: One-Punch Man
Age: At least 300,000,000 years old
Classification: Monster, King
Lifting Strength: Unknown
Striking Strength: At least Mountain Class
Durability: At least Mountain level (Should be stronger than most Dragon level threats. Stated himself it would take a meteor to kill him. Didn't mind the existence of the missile strikes that hit him)
Range: Hundreds of meters via sheer size
Standard Equipment: None notable
Weaknesses: None notable