The creative team of Magic has always had issues with sea-based creatures, as the game, flavorfully speaking, occurs on a battlefield. Summoning sea creatures never made much sense. Odyssey block removed several of the core races for a block to try and shake things up, and Merfolk went away and didn't return until years later. Many players had always liked Merfolk and were unhappy with their absence. In the end, R&D decided that in this case the overall happiness of having Merfolk in the game overrode the inconsistency of the flavor. The creative team also looked for ways for them to make sense; the merfolk of Zendikar block, for example, have finned legs rather than fish tails. 
Humans speculate that merfolk are Atlantean humans that adapted to the ocean; merfolk speculate that humans are merfolk that have adapted to live on land.
Gaea's Skyfolk were an indirect product of the Phyrexian Invasion. As the whole world mobilized its reserves, even ancient adversaries cooperated together, lending their diverse skills in the service of the common good. The Skyfolk were partially of merfolk and partially of elven origin, combining the unique features and skills of both races into one. The detailed description of their abilities is not available, but the major one was the ability to fly. The details of the creation of these prodigal creatures are, however, not known.
Merfolk of Lat-Nam
The Merfolk of Lat Nam appear only in the novel- The Gathering Dark- and appear to be a speechless species that is highly aggressive toward humans and attacks merchant ships on their way through the area in order to pillage them.
Notable Otarian merfolk
Razorfin Hunters are a bizarre crossbreed of goblins and merfolk found beneath the seas of Dominaria. Little is known about them, although it is likely they were the result of mutations caused by the Rathi Overlay.
Vodalian Merfolk lived in the Voda Sea, being particularly prominent in the waters around Sarpadia. They once had an extensive empire, but following the climate changes that took place following the Sylex Blast, much of their territory was overrun by the Homarids. However, some vestiges of the Vodalian Empire, such as the colony of Etlan Shiis, survived the Homarid attacks and still existed at the time of the Phyrexian invasion.
The name of the empire is derived from Russian (and generally Slavic) word "voda", meaning "water". In accordance with this, the known Vodalian characters mostly have Russian-themed names.
Notable Vodalian merfolk
Lorwyn and Shadowmoor
In the dual plane of Lorwyn/Shadowmoor, merfolk are known as merrows. In Lorwyn they control commerce and trade routes, which takes places through the plane's extensive rivers. However, in Shadowmoor, they are murderous cutthroats and thieves who steal from the other races of Shadowmoor who live along the rivers, particularly the kithkin. It should also be noted that the merrow of Shadowmoor have abandoned shaping the tides, leaving some rivers to dry up and others to flood.
The Lorwyn merrow society is divided into "schools", with each school performing a different type of service to the transport and commerce system. The known schools are:
These names are just the "land-walker" language closest imitations of the true names (e.g. Paperfin being Paupurfylln in Merrow). Each school is led by a "reejerey".
Notable Lorwyn merfolk
Present only in Shadowmoor, selkies are unusual merfolk that are different from any others of their kind. Rather than fish, selkies' lower bodies resemble seals, giving them an appearance similar to centaurs. Selkies have six limbs: Two pairs of seal-like flippers on their lower bodies and two arms on their torso with flipper-like hands. Selkies' faces combine human and seal features; they have white eyes with no pupils and long, stringy black hair.
Selkies live in the network of rivers and ponds that stretch across Shadowmoor, but they feel as though they belong in more open waters like the oceans of other worlds, a feeling that gives them a wistful and tragic side. This makes some selkies cold and spiteful towards the land-dwellers who can travel as they please throughout the plane.
Saprazzan Merfolk were once native to Dominaria, but were taken to Mercadia by the dragon engine Ramos in order to ensure the species' survival during the Brothers' War. The refugees built a floating city called Saprazzo, from which they take their name. They were led by the Saprazzan Vizier and worshipped the Power Matrix as a holy relic, but relinquished it to the Weatherlight crew when they learned it was part of the Legacy. They then aided the crew and the Cho-Arrim in overthrowing the Kyren regime.
Rootwater Merfolk lived in the rootwater that nourished the Skyshroud Forest. They had a hideous appearance that resembled an eel rather than a fish and were also extremely aggressive, attacking anyone they spotted travelling on the forest floor. They lived on Rath until the Rathi Overlay, when the rootwater planeshifted to Keld along with the rest of the Skyshroud. They would have all frozen to death in Keld's hostile climate, were it not for the intervention of Freyalise, who created a magical ward on the forest to keep it warm. Whether the rootwater merfolk survived the weakening of this ward during the events of Time Spiral is unknown.
For millennia, merfolk were thought extinct on Ravnica, as the ocean was covered by the sprawling city and eventually faded from memory. However, massive sinkholes enabled them to resurface, and many have joined the Simic Combine and risen to leadership positions. The current highest-ranking speaker for the Combine, Zegana, is a merfolk.
The two-legged tritons of Theros are partially amphibious. They can breathe air for several days at a time but must spend time in water to keep their gills soft. Tritons worship the sea god Thassa above all others and work to do her bidding.
Tribal - Merfolk
- Lorwyn block featured three Tribal - Merfolk cards, which could be fetched by Merrow Harbinger:
- Stonybrook Banneret makes Merfolk spells cheaper to cast.
1/1 Blue Merfolk Creature tokens are created by: