Mutant human was a variant of human which existed in a time period in the future relative to Clara Oswald's home time. Saibra was identified as a mutant human, possessing the ability to change form by replicating living cells she came into contact with. (TV: Time Heist)
The "C-RAD" (which stood for "cellular remission and decay") were a type of mutant humans, coloquially known as "Ugly Mutants". Xais was the last of them. (PROSE: The Romance of Crime, AUDIO: The Romance of Crime)
Criminals Eddie and Charlie Nisbett were also mutants but their mutation was recessive, which is why they were permitted to live, unlike many other mutants that were killed by "normal" humans. (PROSE: The Romance of Crime, AUDIO: The Romance of Crime)