>>27584510A sea lion is a perfect candidate for a mermaid-themed Pokemon, since its natural physiology makes it look like it already has a mermaid tail.
You'll notice that Popplio's final evolution does indeed look like a mermaid, but it still retains somewhat more or less the natural body shape of a sea lion.
A sea lion's physiology allows for a creative and cartoony mermaid design, while staying true to the sea lion's natural form and not radically altering to the point where it doesn't look like a sea lion anymore.
