Merpeople were inhabitants of the nation/state known as the Bight of Calormen in the Great Eastern Ocean. They are described as traditional merfolk, with the heads, torsos, and arms of human men and women, and long fish tails below the waists.
In one illustration by Pauline Baynes, they are drawn in a manner similar to medieval depictions of merfolk, with long green tails, pale skin, fins running down their backs, blonde hair, and adorned with seaweed wreaths, pearl necklaces, and wielding tridents.
In the 2005 film, they are depicted with brown-black hair, tan skin, and silvery fish tails with pink fins. They don't sing at the Pevensies' coronation, but swim, leap, dive, and laugh in merriment.