Jeremy
Explorer
A secondary mutation is a second power they give to someone to make them interesting.
For instance, Emma Frost, the one time White Queen of the Hellfire Club who never bothered to get a code name seperate from this, has telepathic abilities but no telekinesis to let her play with Betsy and Jean. So instead they said, "Now she can assume a diamond hard body form." Which even made it into the game.
Angel's got one where in his blood heals people now.
But some established characters like I named above, aparently haven't gotten secondary mutations for some reason. Beast's transformation from blue furred person to lion I don't recall if that is secondary mutation or just further side effects from his early attempts to reverse his mutation.
For instance, Emma Frost, the one time White Queen of the Hellfire Club who never bothered to get a code name seperate from this, has telepathic abilities but no telekinesis to let her play with Betsy and Jean. So instead they said, "Now she can assume a diamond hard body form." Which even made it into the game.

Angel's got one where in his blood heals people now.
But some established characters like I named above, aparently haven't gotten secondary mutations for some reason. Beast's transformation from blue furred person to lion I don't recall if that is secondary mutation or just further side effects from his early attempts to reverse his mutation.