Hey /x/, I'm seeking information on transfiguration

When my sister was younger, she was obsessed with becoming a mermaid. She would watch all these tutorials on YouTube, which were, of course, bullshit. But they implated a seed of curriousity in me. Is there any legitimacy to transfiguration? Are there any rituals one can do to change their form to that of another being? I know various cultures, such as the Navajo, belive it's true because of their myths and legends.

Some info on mermaids would also be appreciated (pic semi-related)

A spirit I've once asked about this told me, shapeshifting is quite hard on the material plane, and it's usually done with the help of other spirits. But, for some reason she didn't explain, that's too dangerous and I shouldn't.