A Naga is a snake on the bottom, and a human on the top. It comes from the Sanskrit word nag, meaning snake. It's mostly found in India, but also Bali and Nepal, in Hinduism and Buddhism. The snake most commonly associated with nagas are king cobras. They are usually seen as a deity of oceans and mountain springs, and very often are associated with water. Check out the Mahabharata for Indian stories of naga deities. In Buddhism, a naga named Mucalinda is said to be the protector of Buddha.

The mermaid is a fish on the bottom and a human on top. In European seafaring folklore, the mermaid is said to inhabit the dangerous waters of the ocean, and can sing an entrancing song to entice sailors to their deaths. Supposedly they eat the men they catch. Do not confuse mermaids with sirens. Sirens are more like fairies, and might have legs and feet. The first mermaid story was around 1000 BC, in a greek myth about a goddess turning into a fish/woman to hide from humans.