Psaro, also known as Psaro the Manslayer (most commonly in his monster form), is the main villain and one of the most important figures of the world of the Famicom videogame Dragon Quest IV (localized as Dragon Warrior IV for the NES), its remakes, and the main series' spinoffs ever since it first came out in 1990. He is a demon that passionately hates humans and ever since he lost his lover to them, holds a lethal grudge to the race.
Psaro at first appears fairly humanoid, though his red eyes, pointed ears and vast powers would no sooner reveal him for what he really is. He has long silver hair and wears a dark outfit, a single spiked shoulder pad that appears to be made out of bones and a long fur cape. Upon using the Secret of Evolution he turns into a brown look-alike of Estark who gradually loses his arms and head as the combat progresses to steadily morph into a green horned vicious monster with a second face on his torso.
Psaro hates and is in turn reviled by humans, revered by demons and the inhabitants of Rosehill. Rose, his elf love interest who he himself gave her her name in honor of the town that took care of him, was attacked by humans for the ruby tears she sheds. Psaro protected her by concealing her in a small chamber within a tower that he built far away from human settlements that could only be accessed by playing a flute. He held the town of Rosehill in great esteem and helped it grow as others looking for shelter from humans relocated there. Psaro paid frequent visits to Rosehill in order to see Rose; he told her of his plans to destroy humanity to free the world for demons, and continued on with righteousness in spite of her pleas to stop and try for coexistence.
Towards the end of the game, Rose is murdered, causing Psaro to go insane. He loses what little inhibition Rose had given him and uses the Secret of Evolution to transform into a monster, destroying his personality as an unavoidable side-effect. The only thing he remembers is his desire to destroy humanity. In the end, he is defeated by the "Chosen Ones".
In the Playstation and DS versions, after the main story is complete, an alternate choice can be made in which the hero/heroine can seek out a mystical flower from the holy tree Yggdrasil that can be used once to bring back the dead; the hero/heroine can use it to bring back Psaro's beloved Rose. The hero can then escort her to Psaro's domain where she can restore Psaro to his senses and make him return to normal. After this Psaro will show regret for his actions and gratitude to the human heroes for saving his lover, he will then join them on their crusade to destroy the true villain of the story, the one responsible for Rose's death and Psaro's corruption, Aamon.
- He appears as a legacy boss in Dragon Quest IX and his map can be obtained randomly from Dragonlord when he reaches level 16 at least.
- In the Dragon Quest Monsters Joker series, his final form appears and it's a Rank X monster of the Unknown family. It can be synthesized with a Baramos and either a Drakulard or Khalamari.
- He appears in his humanoid and monster forms in as the final boss of the main storyline. Later, his monster form appears as one of Nokturnus' finest warriors in Dragon Quest Battle Road Victory as the final opponent that the player must face in the Dragon Quest IV story arc.
- His appearance, behavior and motives are reminiscent of those of Sephiroth, a villain from the equally veteran RPG series from Square, Final Fantasy. They are often portrayed together in unofficial fanworks, mainly in Japan
- He is thought to be the first boss with a metamorphing animation in videogames.