Mojo, also refered to as the Spineless One, is an alien supervillain and enemy towards the X-Men, and Excalibur who is in many ways a parody of mass-media and reality television - he has appeared both in Marvel comics and in the X-Men animated series and was an antagonist in both roles..
Part of a race of spineless beings, most of the denizens of what would come to be called the Mojoverse were slowly driven insane by waves of energy from another space/time continuum. It would take centuries (of their time) for them to discover the origin of these transmissions. Mojo's race did not evolve much because of their inability to stand upright, until a scientist named Arize developed exoskeletons that allowed a fast technological revolution. However, some members of the race refused to use them and called themselves the Spineless Ones. Instead, they used motorized platforms to transport their bodies. They also became the rulers and demanded a race of slaves to do all the tasks they could not or would not do. Arize created the slaves, humanoid beings, using genetic engineering and basing their appearance on the "demons" of the Spineless Ones' nightmares - which were actually the characters of TV broadcasts of Earth-616, somehow scattered throughout their timeline and perceived by the Spineless Ones. Unknown to the Spineless Ones, Arize secretly planted the seeds in their genetic make-up to eventually turn on their masters and rule Mojo-world justly. Arize was banished by the Spineless Ones when he refused to build weapons for them.
The power structure of this world was based on the television industry and Mojo became their leader by controlling it, along with the slave trade. Mojo named the world after himself; 'Mojo World' and the universe the 'Mojoverse'. His followers, including Warwolves (vaguely dog-like metallic beings with the ability to kill people and take over the remains of their bodies as skins), became known as Wildways. Mojo also has a chamberlain, an android named Major Domo, who oversees Mojo's financial records and relays Mojo's commands to his servants. Major Domo is completely loyal to Mojo, but often obeys with sarcastic comments towards Mojo. Major Domo is sometimes assisted by the panicky female android Minor Domo.
One of the slaves Mojo had ordered, Longshot, became one of his best stunt performers. However, Longshot disagreed with the rules and led a rebellion (most likely motivated by the death of Jackson, a fellow stuntman). He was captured and had his memories removed, but managed to escape to earth, followed by bounty hunters. On Earth Longshot found allies in the form of Dr. Strange and the human stuntwoman Ricochet Rita and they managed to defeat Mojo, driving him back to his own world. Longshot, his friend Quark and Rita returned to the Mojoverse to free their fellow slaves. Longshot's mission failed and they were captured by Mojo. Longshot was brainwashed again, while Rita was tied to the bow of Mojo's world-traveling ship, serving as a guide. In an alternate future, Mojo had Rita transformed into an insane warrior/mage named Spiral and she was sent back in time to capture or kill Longshot.
Mojo became interested in Earth and captured Betsy Braddock, the mutant known as Psylocke, who had been blinded, and implanted bionic eyes in her original body. Much later it was learned that the eyes Mojo gave her were in fact interdimensional cameras allowing Mojo to record and broadcast all she could see. Psylocke was rescued by the New Mutants, and she aided both the New Mutants and the X-Men before officially becoming an X-man herself. It was later revealed that Betsy's body retained robotic eyes, as her consciousness was put into the body of a comatose Japanese assassin.
Shortly afterwards, Mojo sent Longshot to Earth where he joined the X-Men as well. Mojo had planned on enslaving the X-Men by turning them into children, but the New Mutants managed to free them and together they forced Mojo to flee again. Still Mojo found out that the adventures of the X-Men shot the audience level to top ratings; which increased his political power.
Mojo also manipulated the X-Men member Rachel Summers into working for him, but she soon escaped after realizing she was little more than a prisoner. Further setbacks began as Psylocke's broadcasts stopped when the X-Men were presumably killed by the Adversary, but Mojo wanted to have footage of the X-Men to improve his ratings, so one of his assistants came up with a possible solution; create versions that could be controlled. Many different versions of the X-Men appeared before him, but he considered them all failures and ordered their death except the X-Babies. The X-Babies were an immediate success but they rebelled almost immediately and escaped taking Rita with them.
Mojo reappeared along with an alternate version of Jubilee. Mojo had kidnapped Jubilee, taking her to the "Big Crunch", the end of time where all matter would collapse. Jubilee agreed to be Mojo's slave if he wouldn't interrupt the Crunch. Older and renamed Abcissa, she kidnapped her younger self and took her to the Crunch. Wolverine appeared with a missile and defeated Mojo. Since Jubilee refused Mojo's offer, Abscissa's existence was nullified.
Eventually, Longshot returned to free the slaves and deposed Mojo, with the help of Mojo's upright, more human looking (although yellow) clone Mojo II. However, Mojo II turned out to be just as bad as his predecessor and Longshot had to defeat him as well. After Mojo II was ousted Mojo reclaimed his position and became the leader of Mojo World once more.
Mojo never learned from his mistake concerning the X-babies and would create more X-Babies, who also rebelled. Eventually all of the X-Men had X-Baby counterparts on Mojo-world, and they all rebelled against Mojo and fled to an area he could not reach. He also created the Mighty 'Vengers (child versions of the Avengers) to finally stop the X-Babies once and for all. Of course, the Mighty 'Vengers, being essentially of the same moral fabric as their adult counterparts, turned on Mojo and defeated him. Finally, he created toddler versions of all of the Marvel supervillains. These characters are still employed by him although some of them were more intelligent and may even compete with him for dominance of Mojo-world.
Recently the Exiles made a deal with Mojo: in return for Longshot's help, Mojo receives broadcasts from all over the multiverse through the Exiles' crystal palace. Mojo reappeared along with the previously vanished Nocturne and Juggernaut, using a "Jean-Bomb" to turn the X-Men into babies, and after his defeat, a guilt-ridden Juggernaut decided to try to become a better person and left with him.