Pluto

"Time to go home, Herk"

- Pluto

Pluto is a main character in the episodes The Rise of Pluto and The Fall of Acheron, as well as his own manga series. Built by Shadow in order to help Astro evolve, he is shown to be heartless and careless at times (such as his first interaction with Astro after he was nearly destroyed) with the sole thought of defeating Astro and furfilling his purpose.

The Pluto episodes are actually a 2003 remake of The Greatest Robot in the World story arc, and Pluto is actually the 2003 version of the character Bruton in the 1980s series. Because of this, the characters Pluto fight are also remakes (e.g. Epsilon is actually Photar), although the final character that defeats him is different (he was defeated by Acheron in the 2003 series, Bora in the 1980s). His purpose and his creator also differ dramatically throughout the two series.

Trivia
Pluto's horns have always been extremely large as well as his weapon and important feature. His weapon in the 2003 series, however, is a set of homing missiles located in his chest, and his horns were never mentioned or referred to during battle.