Mer-Monster

Mer-Monsters (Mutants in Season 5) are the monstrous counterparts to the merfolks of Andros. In Season 3, a great majority of the mermaids residing on Andros were forcibly turned into mer-monsters by Valtor as a way to kickstart his conquest over the Magic Dimension after he broke through Andros' portal to the Omega Dimension with the Trix. In Season 5, both mermaids and tritons were turned into mutants due to Tritannus' dark magic.

Season 3
Having been forcibly transformed, the mer-monsters' eyes change color and they become much more hideous compared to their original appearances.

There are two types of mer-monsters: the reptilian ones and the humanoid ones, but both are equally as dangerous. So far, mer-monsters of these types are only female as Valtor had only forcibly turned mermaids during his conquest of Andros.

Season 5
The Mutant are Tritons (sometimes mermaids) that were turned into monsters by Tritannus' pollution-magic. Their upper body has an ill colored skin tone with many wrinkles to their face and transparent flaps beneath their arms. They have a reptilian appearance and is warrior type, with a shield attached to their right hand. They have yellow eyes and sharp teeth.

In "The Mystery of the Abyss", it is revealed that Politea can turn the jellyfishes into mutants.

Personality
Under the effects of Valtor's dark magic, mer-monsters was aggressive and wild. They attack anything that crosses their paths and do not stop until either they are subdued or person or object they are attacking has been defeated. Fortunately, the mer-monsters return to being kind and docile once Valtor's magic is dispelled from them, reverting them back into their original state as mermaids.

Magical Abilities
Alongside their physical combat skills, intense strength and durability, there is one instance in the series in which a mutant manifested a magical blast of energy.

Trivia

 * Unlike the mer-monsters in Season 3, all Mutants are alike, there is no difference between mermaid mutants and triton mutants.