Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.

 

Watching mermaid’s movies or reading stories in an entertainment way are fine. `A’isha, the Prophet’s wife (may Allah be pleased with her) used to have a toy of a horse with 2 wings. But to believe in their existence as holy and sacred and have some sort of praise for them to that will make it haram (prohibited).

 

Almighty Allah knows best.