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)