Merfolk is a common name for various races of fish-people appearing in science fiction and fantasy. A female merfolk is often called a mermaid or siren, while a male is a merman or triton. The race as a whole may be called merpeople, the Mer, mermen (this name was more popular in 80's) or some other name specific for a fictional world. Although "mere" means "sea" in Old English, there are also river and lake merfolk.
- Traditional mermaids with fish tails and human bodies from waist up. They are often viewed as a female-only race. Disney's Ariel and her people from The Little Mermaid are this type of merfolk.
- All- or mostly-human beings, but breathing and living in water, such as Slavic Rusalka or aquatic elves of Dungeons & Dragons. Even if they look human-like, they often can swim faster than any human, can communicate with sea life, and have other water-themed powers.
- Beings with tail of a water creature and other physical traits of this animal. For example, merfolk of this type can have human-like faces, but scaled skin, fins on various parts of their bodies and shark-like eyes. Animals beside bony fish that contribute to this type of merfolk are sharks and rays, cetacean, crustacean, cephalopod, mollusk, snakes, turtles. Merfolk of old World of Darkness and Magic: the Gathering are of this category.
- True anthropomorphic fish, often with traits of amphibians. This type of merfolk is much influenced by H.P. Lovecraft's Deep Ones: anthro-merfolk are often an evil race, or at least dangerous and alien to land-dwellers. Story motives include forced or religious transformation, mutation, culture dating back way before humans. Warcraft's murloc and naga are this kind of merfolk.
Merfolk settings sometimes include fictional animals resembling land animals modified to live an aquatic life, generally with a fish tail. The most common species for this is the horse. The aquatic horse is called hippocampus.
 Merfolk and furry
All types of merfolk have some following in the furry community, but the less animal traits they have, the more often they are considered not furry. Even if not considered furry, merfolk are usually strongly connected with underwater creatures and tend to exist alongside with anthropomorphic animals. For example, Disney's Ariel has large supporting cast of talking sea life.