You may have noticed, in my post on mongrelfolk, that the Triton's Ally feat had the prerequisite "Seafolk Heritage (Triton) or Triton-Descended Mer". Conspicuously, Triton-Descended Mer was absent from that list, because it is not a mongrelfolk feat.
Here's the story: tritons and merfolk live in close proximity to one another, and frequently interbreed. But there is no template or race for half-merfolk triton or half-triton merfolk. Instead, they are, mysteriously, so similar that the offspring of a triton and a merfolk is either a triton or a merfolk. These offspring then have the option to take the Mer-Descended Triton or Triton-Descended Mer feats, as appropriate.
Mer-Descended Triton [Racial]
There is more Merfolk blood in your lineage than that of most Tritons.
Prerequisites: Triton race, first level only
Effect: Your type becomes Humanoid and you lose the Native subtype. Your Outsider hit dice become Humanoid hit dice.
You lose your Darkvision but retain Low-Light Vision. Your Base Attack Bonus from racial HD drops from +3 to +2. Your base Reflex save from racial HD remains +3, but your base Fortitude and Will saves drop to +1. Your skill points from racial HD drops from 8+int per level to 2+int per level.
Additionally, your Level Adjustment drops from +2 to +1.
Triton-Descended Mer [Racial]
There is more Triton blood in your lineage than that of most Merfolk.
Prerequisite: Merfolk race
Benefit: Your type becomes Outsider and you gain the Native subtype.
You gain Darkvision 60 feet.
The sea races have also occasionally interbred with various surface races races, to various effects, including the following feats:
Part of That World [Racial]
Increased interactions and interbreeding with other races (particularly yuan-ti and the rare selkies) have given some merfolk the innate ability to, with practice, work minor transformations on themselves.
Prerequisite: Merfolk race
Benefit: An innate talent for magic grants you the following spell-like ability at a caster level equal to your hit dice: 1/day - alter self.
Special: You may select this feat multiple times. Each time you do, you may use the spell-like ability one additional time per day.
Land Legs [Racial]
Some seafolk, after spending a great deal of time on land, develop the strength and ability to stand and move better, allowing them to move faster on land.
Prerequisite: Dex 17, Dash, Aquatic or Water subtype.
Benefit: If you are wearing light armor or no armor and are carrying a light load, your land speed is an additional 5 feet faster (for a total of +10 with Dash). Land Legs has no effect on swim, fly, or burrow speed.