A wildcat cartridge is a term often used for modified versions of already commercially-produced ammunition. They have been created by enthusiasts and professionals alike, in order to alter or improve the performance of the original.
It's unknown where exactly the term originates from. Frank Barnes, in his famous work Cartridges of the World, notes that these custom wildcat loadings have "...been around for for a long time; at least 100 years." Such creations have probably existed since the dawn of modern cartridge ammunition.