From the Super Mario Wiki
Mermaid Yoshi is one of Yoshi's several transformations that appears in Yoshi's Woolly World and Poochy & Yoshi's Woolly World, specifically in Rollin' Down the River and Frozen Solid and Chilled. The form allows Yoshi to dive with , but Yoshi can also jump higher and climb waterfalls. In addition to this, Yoshi also magnetizes beads towards him.
Mermaid Yoshi has the head and upper body down to and including the saddle of a regular Yoshi, but from this point downwards the Yoshi has a tail made of sequins to give it a scaly and shiny effect. The fins of the tail are rounded and almost heart-shaped. The color of the sequinned tail depends on which player the Yoshi is, with player one receiving a red tail and player two receiving a royal-blue tail.