Zoids: Chaotic Century and Guardian ForceEdit
Main article: Zoids: Chaotic Century
Although there is no mention of any Zoid known as an Ultimate X in these series', when an Organoid merged with a Zoid it produced very similar results: increased strength, speed and stamina, as well a degree of sentience (this, however, is the sentience of the Organoid in the Zoids, not the Zoids themselves). There is speculation that the Organoid System seen in Zoids: New Century is in fact the evolution of the original Organoids seen in these series', permanently fused with a Zoid. Although this explains the apparent sentience and even the personality of these Ultimate X's, there is no proof to show that these are not two completely different systems.
Zoids: New CenturyEdit
Main article: Zoids: New Century
In this series, Ultimate X's were referred to as legendary Zoids, having never been found before and believed by most to be just a rumor. The Liger Zero, initially thought to be worthless as no one could pilot it, was found to be one of these legendary Zoids. Upon closer inspection during a maintenence exam, the inside of Bit Cloud's Liger Zero was found to have a "black box"; an integrated Organoid System which gave it the ability to "learn" from past battles, therefore giving an increasing advantage over any normal Zoid it would have ever fought in battle, as counter tactics could be produced very quickly from the Zoid itself. Ultimate X Zoids also possesses a sense of sentience far past that of any other Zoids. Some instances where this is evident was when the Liger Zero laughed at Bit Cloud, and when Vega Obscura's Berserk Fury refused to stop fighting, against Vega's wishes. In addition, Bit's Liger Zero would sometimes suggest special attacks for Bit to utilize in battle, such as the Seven Blade Attack and Strike Laser Claw.
When two Ultimate X's fight each other, they are generally evenly matched, as they counter each other's attacks and increase in power and speed rapidly as the battle progresses thanks to the Organoid System within them. This type of situation was only seen once, during the battle between Bit's Liger Zero and Vega's Berserk Fury.
It was somewhat implied that Leon Toros' Blade Liger might be an Ultimate X, but that Leon and the Blade Liger just weren't capable of bringing out his Blade Liger's full potential, despite their high combat skills. However, there is no concrete evidence to support this.
Zoids: New Century mangaEdit
In the New Century manga, the "secret" of the Liger Zero is that it has "unlimited potential". Although this is the same terminology used in the anime, the term Ultimate X is never used. (Either for the Liger Zero or the Berserk Fury). Likewise, the black Box is not mentioned.
Zoids: Chaotic Century mangaEdit
In the Chaotic Century manga, like the series, there is no mention of Ultimate X Zoids. However, the progression of Zoids is worth mentioning, Van's next Zoid after the Blade Liger is a Liger Zero, and Raven's Zoid after a Geno Saurer is a Berserk Fury. In Van's instance, the Zoid Core of his Blade Liger is removed and placed into a Liger Zero. In Raven's case, the Berserk Fury is a replacement Zoid for his destroyed Geno Saurer. He does not pilot a Geno Breaker. Due to featuring many of the same characters (and importantly, Organoids) as the anime, there is the same speculation around the manga as there is around the anime. However, the Chaotic Century manga does not appear to be explicitly in the same timeline as the New Century anime. (New Century references the Chaotic Century anime only, not the manga).
There is no mention of Ultimate X Zoids in the Battle Story. There is, however, some mention of an "Organoid System." Despite the name, this system is not to be confused with the Ultimate X. Although the abilities of the Organoid System in the Battle Story mirror some aspects of Ultimate X Zoids, there is no established link between the two.
The HMM Liger Zero and Berserk Fury both come with their respective New Century pilots (Bit and Vega). Despite this, and other anime similarities, there is no Black Box on their Zoid core (the cores are identical to "regular" HMM Zoids). Official HMM sources, such as the Kotobukiya blog, do explicitly state these Zoids are Ultimate X's.