Lawsuits
In 2001, 20th Century Fox sued Marvel, Tribute Entertainment, and Fireworks Entertainment for breach of their licensing agreement and false advertisement. Fox stated it had exclusive rights from Marvel to develop the X-Men property, and anything similar was an infringement. Fox claimed that Mutant X was too similar to X-Men, and Mutant X was being advertised as an "X-Men replacement."
Marvel counter-sued Fox, saying that the two were dissimilar and asking the courts to allow Mutant X production to go forward. Production was allowed, as long as X-Men material was not used in the promotion of Mutant X. Apparently, the title "Mutant X" itself was deemed too close to "X-Men" to be effectively leveraged.
In 2003, Fox and Marvel resolved their differences in a confidential settlement of their suits. Meanwhile, Fox continued to pursue their case against Tribune and Fireworks. Tribune sued Marvel for fraud and breach of contract, claiming Marvel encouraged Tribune to connect Mutant X to the X-Men, misrepresented what they were getting in their license, and caused millions in losses due to the need to alter story lines and characters to ensure the mandated distance between Mutant X and X-Men, as well as fighting Fox's litigation.
Read more about this topic: Feral (Mutant X)