The much smaller mermaid in the pic is a true mermaid, one whose blood has the power to heal injuries and sickness, extend lifespans, and provide immortality in large enough doses.
She about 9 feet tall from head to tail tho basically human scaled. A lot of that length is in the tail. So you can imagine how big the false mermaid grew to be.
Yes the true mermaid is missing an arm. XD Whether that was before her capture or after tho is yet to be determined. The false mermaid is highly protective of the true mermaid and kills anyone who tries to get near her. essentially ending the powers that be's attempts to harness her blood for their own benefit.
Given that her bloods potency cant be reproduced via cultures they opted to try and clone her only to have the clone rapidly grow into a twisted abomination. Even beyond her intense strength as well as the cutting power of her claws and razor sharp teeth, shes capable of spitting a pressurized stream of water that's capable of cutting through even armored opponents with ease from a distance.
They had to give up trying to destroy the false mermaid as any weapon with the strength to kill her would have surely have destroyed the far more fragile true mermaid who is never left very far.