This Lora can remove the arms by trigger word armless, and remove the shoulders too by trigger word ampshod.
Don't you feel strange when you see a fish with human hands, or a snake with human limbs or human skin? The lamia and mermaid are not human beings, not even apes, so they should not have human arms. It would be best if there were no neck, but I can't find enough monster girl images without neck.
No shoulders:
Positive: armless, ampshod
Negative: shoulders,arms
Snake girl with [colored] ventral scales and [colored] dorsal scales:
Positive: armless,ampshod,lamia,[colored] scales,(scales on neck:1.2),(ventral scales:1.2),[colored] ventral scales,
Negative: shoulders,arms,(human skin:1.2),nipples,
Mermaid with flipper arms(like dolphins and seals)
Positive: armless,mermaid,(fins:1.2),(shoulder fins:1.2),flipper arms,
Negative: groin,knees,
Furry mermaid with bodysuit(for lamia is similar)
Positive: armless,mermaid,fins,gill,furry,coiled,horns,gimp mask,
Negative: shoulders,arms,(human skin:1.2),groin,knees,
Feral snake girl(in some way this just an animal)
Positive: armless,ampshod,lamia,scales,[colored] scales,[colored] ventral scales,
full body,no humans,
Negative: shoulders,arms,(human skin:1.2),
A lot of the training images are the same as the model "latex mermaid" in civitai because the images are from the same artist https://www.pixiv.net/users/22732076