From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Ultra Boo is a Majin.
Dragon Ball Z
Ultra Boo appeared briefly in a few panels of the manga and an anime flashback when Kid Boo absorbed South Kaioshin and transformed into a taller and incredibly muscular version of the creature. In addition, his antenna also grew significantly larger. In this form, his voice also changed from the previously high-pitched voice of Kid Boo, though he still did not talk much (apart from yells and grunts similar to Kid Boo). After a second absorption of the plump and good-natured Grand Kaioshin, Ultra Boo tranformed into the deceptively harmless and child-like form first seen in the series. As no name was given for this short-lived form, it has been coined Majin Boo South Supreme Kai Absorbed, Mega Boo, Buff Boo, Huge Boo, and sometimes Super Kid Boo by fans.
Dragon Ball GT: Final Bout
When Kid Boo buffs up into a form similar to Ultra Boo when he charges energy or uses specific combos in Dragon Ball GT: Final Bout.
Dragon Ball Z: Budokai Tenkaichi
This form is sometimes confused with the Super Boo form, including Super Boo's biography in Dragon Ball Z: Budokai Tenkaichi which wrongly identifies the Super Boo form as the form the original Majin Boo took after absorbing South Kaioshin. After everyone was freed from inside of Super Boo, however, he did change back into this form for a short time before he transformed into Kid Boo.