Medieval cavalry was not so much used in frontal attacks, which could be blunted with relative ease, but to try and flank the enemy.
Using the word "mutant" implies some knowledge about genetics and genes. However, "mutation" is sourced from the Latin word "mutationem", and existed as "mutacion" as early as 13th century Old French. So the concept of "mutant", in the meaning "changed", is not new, despite the mechanism of change - DNA - was known a full 700 years later. Hence, the use of the word in a fantasy setting, though having a modern air to it, is justified in a fantasy setting.