Julián is a Mermaid

While riding the subway home from the pool with his abuela one day, Juli n notices three women spectacularly dressed up. Their hair billows in brilliant hues, their dresses end in fishtails, and their joy fills the train car. When Juli n gets home, daydreaming of the magic he s seen, all he can think about is dressing up just like the ladies in his own fabulous mermaid costume: a butter-yellow curtain for his tail, the fronds of a potted fern for his headdress.

Synopsis from Open Library.