Mermaids are winged magical creatures with the lower part of a fish and the upper part of women. There are mermen, who are also known as Tritons. They have magical abilities and are look like larger winged versions of Water nymphs without skin flaps under the arms. Mermaid society is found in the realm of Andros. Mermaids have powerful voices and the ability to fly.
The mer-folk have wings and can fly. Mermaids, like Fairies and Witches may have to attend schooling to learn their Mermaid magic, or it could be that they are able to master their powers by themselves and that older mermaids teach other mermaids how to use their magic like some traditional societies on Earth. Some mermaids have specific powers like Kalia, mermaid of flowers and Nesia, mermaid of sweet song.
By nature, mermaids are gentle creatures and they rather help others but there are seasoned warriors such as the Mermaids who guard the Omega Dimension and Princess Tressa who was trained to be a warrior.There is a a magical link between Andros and the oceans of other planets in the Magical Dimension which mermaids travel through which explains why some mermaids may possibly have existed at one point on Earth. Now that the Earth's magic has been restored since the events of Season 4, it could be that mermaids may come back to Earth through the magical link between the Ocean of Andros and the oceans of Earth, similarly to how Tritannus found himself at the site of an oil spill on Earth.
In Season 3, Aisha was summoned to Andros by Tala and they both swam into the ocean on Magix to emerge on Andros. According to Aisha and Tressa, the depth of the Ocean of Andros does not reach to an end and does not have a bottom. Mermaids are to be adept at traveling the underwater currents of Andros.
Mermaids that were changed into monsters by Valtor and the Trix when they broke out if the Omega Dimension. Valtor puts his mark on them to turn mermaids into dark mermaids. Their eyes change color and their faces get more hideous. There are two classes, the reptilian and the humanoid class, both are dangerous. As this was a transformation and corruption brought on by Valtor to bring out a Mermaids evil side this may be a transformation, devolution, or evolution or just another mermaid's aspect.
The mermaids of Andros are currently ruled by Queen Ligea and her daughter and successor Princess Tressa, and seem to have a matriarchal society. Their messenger and ambassador is Tala. Mermaids tend to stick together in close groups called schools. And they have their own language which people from Andros seem to understand. The mermaids and people of Andros have a great relationship and many mermaids are friends with Aisha and regard her as a heroine.
The Guardian Mermaids have the important task of guarding the portal to the Omega Dimension. All Guardian Mermaid guards look alike with dark skin, green hair, purple tails, leathery small Winx level-like webbed wings from their back which allow them to fly, wearing armor and carrying spears from which they shoot magical blast (similar to Warrior Fairies). The Mermaid guards that were transformed into Mer-Monsters by Valtor, took on a more reptilian appearance than the other mermaids who looked more humanoid.
In Season 5, the Neptune was introduced as the King of Andros, ruling it's Seas while Teredor rules all the lands above the surface and it was revealed that Ligea is his wife, and that the twins Nereus and Tritannus are their children and the brothers of Tressa. This also means that the male counterparts of the Mermaids are the Tritons.
These were created from the dark magic of the Wizards of the Black Circle used to guard the island of Tir Nan Og so as to prevent people from finding the location of the Earth Fairies or from helping them.
There is a speculation that mermaids are a form of fairy transformation because of their wings and sparkling tails and they share a similar appearance to the water nymphs of Magix. If this is so then that means that a mermaids may be able to transform themselves into land-dwellers and walk on land, but this is unlikely as it has never been mentioned that mermaids were a fairy form.
Another fact which makes this unlikely is that when Valtor invaded the Ocean of Andros, the Mermaids were forced to live in the dangerous seas and could not flee to go on dry land.
PopPixie mermaids are diferent, they don't have wings so they can't fly.They live in a lake that isn't in Pixieville. In The Mermaid's Potion, Amore, Chatta, Lockette, Martino and Fixit were turned into mermaids by Rex.