Mermaids

Mermaids are humanoid species that are native in Neverland, the Enchanted Forest and the Caribbean. Notable mermaids are Ariel and Ursula.

Biology
Mermaids are aquatic creatures with the head and upper body of a human and the tail of a fish. They are able to breathe air while out of water.

Every year when the high tide rises, the sea goddess Ursula grants merpeople the ability to walk on dry land with human legs, until the next tide occurs. Mermaids have the ability to cross worlds, and to open portals and bring objects from one world to another. They can be summoned by Conch Shells. ("Ariel", "Dark Hollow", "Poor Unfortunate Soul")

Ursula, who was a Mermaid before

being transformed into a Cecaelia, is the daughter of Poseidon, who is a Deity; indicating that merpeople can interbreed with Deities. ("Poor Unfortunate Soul")

Nature
Most mermaids come off as cruel and mean. According to Hook, mermaids are vicious and malovent. This is apparent when a mermaid attacks the Black Pearl. They were also known to sing beautifully to lure sailors into crashing with their rocks, such as Ursula singing and later crashing ships to please her father (which makes her upset). They are also liars.

However, others are kind and caring. Ariel fits this when she saves the life of Snow White and Thorin Oakenshield

During first Curse
A young mermaid named Ariel rescues a man named Eric from drowning. Like the original little mermaid, Ariel leaves The Prince before he wakes up. She sees Snow White and later helps her and Thorin escape the Evil Queen and reunites with Erik. Regina later disguises herself as the deity Ursula and gives Ariel legs. The naive young mermaid inadvertently turns Snow White into a mermaid, who can not run. Last time around, Ariel transforms back into a mermaid while rescuing Snow and Thorin, yet again (Ariel")

At another time, the Evil Queen goes to Rumplestiltskin's castle to discuss with him a deal with a particular mermaid. The conversation is cut off when Rumplestiltskin asks her to leave, to which she does. ("Skin Deep")

At one point, Jack has a relationship with a mermaid, but later leaves her ("Heart of the Truest Believer")

After First Curse
Mermaids appear to attack Emma, Bilbo, Jack, Hook and their allies when they are aboard the Black Pearl. Later, when Regina leaves the group in favor of teaming up with the Dark One, Regina calls back Ariel and returns her her voice.